These grainy images of a supermarket security camera record a young child being abducted. A woman leaves followed by the child, who appears distressed. The woman has taken the little girl's umbrella. Outside, the child will vanish, like tens of thousands of others before her. Cai Xing Quan's daughter was abducted on 27 June, 2007. These are the last pictures of her.

MR CAI (Translation): I watch them every day. I study them every day. But there are no results. That day was her birthday. We had prepared a birthday party for that night. She missed her birthday. A happy day turned into a sad day.

Here in Shenzen, a teeming industrial hub, criminal gangs target the children of migrant workers like Mr and Mrs Cai because they are transient, often fearful of the police and whose grievances are rarely treated as high priorities. Mrs Cai was distracted by exhaustion on the afternoon her child was taken and so blames herself for what happened.

MRS CAI (Translation):  Of course, she asked me to go with her, but I didn't. I was too tired and wanted to sleep.

The past two years have taken their toll on Mrs Cai. She doesn't sleep much at night and struggles to stay awake in the day.

MR CAI (Translation):  There is a child missing every single minute in China. There are so many cases. Child trafficking is just like a business. Children become commodities for buying and selling. That's how things are now.

Mr Cai took us to the street near his home where his daughter was taken. The cameras in the shop were working but not the ones out here in the street.

MR CAI (Translation):  That's as far as we've got. It's our last clue. The police surveillance cameras were all useless. They were all out of order.

REPORTER (Translation):  But how?

MR CAI (Translation):  They said the cameras were damaged in the rain. I can tell by the way she coaxed my child away that she must be an experienced children trafficker. She left no trace whatsoever.

China's strict 1-child policy forced Mr and Mrs Cai to flout the law. They already had two sons for which they were fined. But they broke the law again when their daughter was born.

REPORTER:  Does he remember his sister?

MRS CAI (Translation):  Of course he does. He always talks about his little sister.

Cai Feng is the younger brother. The cruel irony is that with his sister gone the family is now legal again.

MR CAI (Translation):   Some children go through a few people's hands. It depends - if she's lucky, she could be sold to a family. If not, she could become a beggar or sell flowers.

Well, I have just learned that minutes after we left Mr Cai's house yesterday, the police paid him a visit. And in fact I have learned subsequently that the police have been round to see him every day for the past few weeks. He's not the only parent being watched, others are too. And the fact they are prepared to speak out to foreign journalists about their plight is, I guess, a measure of their desperation.

SUN HAI YANG (Translation):  I put up lots of posters offering 100,000 yuan, even at the police station.

Sun Hai Yang has transformed his home into a makeshift shrine dedicated to China's missing children. They include his son, who like the Cai family's daughter, was also abducted two years ago.

SUN HAI YANG (Translation):  My son was taken away on 9 October 2007 at 7:30 in the evening. He was playing outside. A child abductor came with a bag full of toys. He was a professional.

Mr Sun knows for sure 3-year-old Sun Zhou was kidnapped because he, too, has chilling pictures of the moment his child vanished. The trafficker can just be seen leaving a toy to entice the child, and then are seen leaving together - all this happening just metres from where his father was busy with customers at the dumpling shop he runs. Mr Sun and his family had only moved to Shenzhen from the north of the country six days earlier. He thought his son would have a better life here.

MR SUN (Translation): I've started feeling numb by now. I came to this city with a lot of hopes. I felt I was finished when my child went missing. There's no hope in my life.

Another hardworking parent who blames himself for the loss of his child.

MR SUN (Translation):  When we run a business, we can't care for our children. Child traffickers target families like us. For those who have good jobs and are well off, they can hire carers. Others can have their parents look after their children. Song Hui, Wang Qingxin, Tan Xiaohu, Zeng Xiaofeng...

The sad roll call of Shenzhen's missing children. A list of almost 200 names - the result of Mr Sun's own detective work.

MR SUN (Translation):  They were generally three or four years old. This one was only two days old when he went missing. He was stolen as a newborn without even having a name. Most of them are boys. As for the proportion, 9 out of 10 are boys.

There are more names and faces on the website he helps run called 'Baby Come Home.' Parents of missing children from across China exchange information and experiences. Some write blogs critical of the authorities, which are usually quickly shut down by the internet censors.

MR SUN (Translation):  My only purpose in doing all interviews and petitions, is to get the government's attention. Otherwise we can't get our children back.

Mr Sun's 10-year-old daughter Sun Yue says she still dreams her little brother will come home. But, after two years, it's getting harder to believe that will ever happen.

SUN YUE (Translation):  I want to tell him to come back home quickly. He can have all the things that he wasn't allowed to eat, like ice-cream and other things.

Traditional beliefs favour boys over girls. Without a real social safety net, many parents rely on the boy to look after them in their old age. The country's 30-year-old 1-child policy has only served to enhance their value. Boys sometimes sell for as much as $6,000, girls for as little as $500.

Missing children simply don't make the headlines here, and it's this absence of that media coverage that's forced parents to take matters into their own hands. They recently produced thousands of packs of cards like these. Each card bears an image of a missing child.

We don't know just how many children have been abducted since the 1-child policy began because the central government refuses to release those figures. But China's state media recently reported that between 30,000 and 60,000 children vanish each year. According to Roseann Rife of Amnesty International Asia, it's a figure that can't be openly discussed.

ROSEANN RIFE, AMNESTY INTERNATIONAL: It's going to be very difficult to get any official statistics because statistics at the provincial level or the national level on trafficking of women or children are classified as state secrets. So people are going to be reluctant to talk about it out of concern that they might themselves end up within the criminal justice system.

Despite the dangers, some, like Mr Sun, are determined to speak out.

MR SUN (Translation): This has become a huge issue for China's reputation. It needs a solution. A big society is made up of many small families. But small families are being destroyed. We spend our time not working but searching for our children. Parents, grandparents and relatives are all involved. Parents abandon their business and go madly searching. That's not good for a stable and harmonious society. I urge the central government to resolve this issue quickly.

Mr Sun has also had several run-ins with the police for refusing to remove the posters and banners that decorate his shop. Two years on, his wife can barely imagine what has become of her precious son.

MRS SUN (Translation): I think about him all the time... ..especially when I hear other children call their mum. When I see some children begging for money in the street, I worry that my child is forced to do that too.

She worries because, in some cases, a snatched child will be sold onto a gang of beggars or thieves, like Oliver Twist. The grief of the parents has been exacerbated by the response of the authorities. These are pictures of a protest broken up by police in Beijing last August. Mr and Mrs Cai were hauled away to shouts of: "How dare you question the government?"

MR CAI (Translation): The police arrested us on that street. We were detained for three hours. They took us to the section where people appeal for help. But we didn't appeal for help. We were taken there forcibly.

Well, after those depressing and distressing interviews in Shenzhen we have come further north to the province of Hebei to meet a group of parents who are taking matters into their own hands. They have been driving around the north of China trying to raise awareness about child abductions. But there's a problem.

YANG ZHENG JIAN, ON PHONE (Translation): We were followed by the police and they were there when we got to Taiyuan.

The police have intercepted their van, detaining them for several hours, before ordering them to return to their home province hundreds of kilometres away. The group those parents were planning to meet still want to see me, though. And, so, a clandestine rendezvous is arranged at a hotel in the provincial capital, Shijiazhuang. They're aware of the risk they're taking because the police have been watching their homes. So, this is the only protest they can make - inside the hotel room. Not all the parents of missing children are poor, Mrs Wang owns a chemical factory, but she's also part of an ad hoc pressure group that includes farmers, labourers and migrant workers. She holds the six of hearts playing card, bearing the picture of one of her two sons, abducted together, in 1995.

MRS WANG (Translation):  Indeed. It's been 14 years. When they went missing, one was five, the other six. They should be young adults now, one is 19 and the other 20.

REPORTER: Does she think about them every day? Does she believe in her heart that she will see them again?

MRS WANG (Translation):  As long as I keep searching, I'll be reunited with my sons.

These four parents say they are victims, too, but the police regard them as troublemakers, saying their actions are creating "social instability".

MR YANG (Translation): I feel the police aren't trying very hard to find our children. Instead, they're trying to stop us. We've formed a group to search for missing children.

The ultimate penalty for trafficking in children is death - a sentence rarely imposed, although these parents would have no second thoughts.

MR YANG (Translation):  Once they've been captured, they should all be shot. Only severe punishment can stop child trafficking and stop the business of selling children for money.

MRS YANG (Translation): They all deserve death by hanging. They should pay the price for what they've done.

MR WANG (Translation):  I don't understand why they use their energy against us instead of trying to catch the traffickers.

 MRS WANG (Translation):  We haven't caused any trouble for our country, or the Party. Why aren't we allowed to search? I don't understand. We don't rely on them. We rely only on ourselves. We haven't asked for a cent from them. We aren't against the Communist Party. But the police have done nothing. We have no desire to keep on living any more.

Amnesty International says their despair is nothing unusual.

ROSEANN RIFE: Unfortunately, what they are meeting is local authorities who are trying to stop them, immediately harassing them, prohibiting them from going to the central government or to higher authorities, trying to keep them from talking to journalists, both domestic and international, about the issue. So, they are really facing human rights abuses themselves in an attempt to find their children.

REPORTER: So it's risky for them?

ROSEANN RIFE: It's absolutely risky.

We made repeated requests to the Shenzhen branch of the Public Security Bureau for an interview. No-one replied. But the Public Security Bureau's website is trumpeting one recent success - the rescue of 60 children following a 6-month operation.

State Television has also begun broadcasting reports about missing children being reunited with their parents. And, just a week ago, Chinese police arrested over 40 alleged members of a child trafficking ring that sold 52 children over two years, reportedly earning nearly US$60,000. What's never mentioned, though, are the many thousands still missing. Mr Sun says he believes Beijing may be concerned about the issue, but that abductions are not a priority at the local level.

MR SUN (Translation): Whatever the local authorities said to us, they did nothing to look for our children.

This week, China's government faced more uncomfortable questions about the kidnapping trade when a question was asked on Dateline's behalf at a regular Foreign Ministry media briefing.

REPORTER: Can you give me a figure for the number of children classified as 'missing' today in China, and does this confirm that the government is facing a huge social problem because of this? Thank you.

MA ZHAO XU, FOREIGN MINISTRY SPOKESMAN (Translation):  China is determined to crack down on trafficking women and children. According to the recent media report, the Chinese police had taken action and rescued a group of abducted people. Such actions by the police will continue. As for the figures you mentioned, I suggest you find out from the organisations concerned.

Despairing of police efforts to find his child, Mr Sun continues to defy the authorities. He's put up thousands of posters, appealing for information about his son.

MR SUN (Translation): But we haven't found the child or the abductor. So I offer 200,000 yuan to anyone who can provide any clue, including the abductor himself, if he can tell me where the child is.

A curious crowd gathers round him. They listen sympathetically.

MR SUN (Translation): Do you recognise this person? She stole this child.

MAN (Translation): No, the face isn't very clear. I'll take one.

MR SUN (Translation): You can have this one with a clearer picture.

MAN 2 (Translation): Did you look for your child?

MR SUN (Translation): Yes.

MAN 2: (Translation): You still haven't found him?

WOMAN (Translation): How long have you been searching?

MR SUN (Translation): Two years.

WOMAN (Translation): Have you found any clues?

MR SUN (Translation):  We know who did it but we don't know where she is.

Most parents I spoke to exist on the hope they will see their children again, despite the attitude of the government and the police.

WOMAN (Translation): She knows her brother's school and his name. She knows all this. I'm sure she'll come back when she grows up.

MR SUN (Translation): I don't think any foul play occurred in my child's case. He's alive. As long as he's alive, I'll find him.

GEORGE NEGUS: Baby, come home. And these are the cards Adrian mentioned there. Pick a card, any card, and it tells the same story - another missing Chinese kid. Not exactly China's best look.

Reporter/Camera

ADRIAN BROWN

Producer

ASHLEY SMITH 

Editor

NICK O’BRIEN

I.T.Support

SHAUN HAYMAN

Translations/Subtitling

JING HAN

Original Music composed by

VICKI HANSEN