Leeds Crown Court heard that the girls, aged 13 and 14 at the time of the murder, inflicted more than 100 injuries, including 80 to the face of Angela Wrightson, 39, in December 2014.
On the day of the attack, the girls, who cannot be named, let themselves into Ms Wrightson’s home and asked if she could go to the shop for them, the BBC reported.
The court was told the girls had previously visited Ms Wrightson, who was believed to have had an alcohol problem, on several occasions because she would buy them alcohol and cigarettes.
Despite the girls giving different accounts of what happened next, the jury heard that Ms Wrightson was forcibly restrained and "brutally" assaulted in 12 separate areas of the home.
Weapons used during the five-hour attack including a coffee table, computer printer, a wooden stick laced with screws, a television set and a shovel.
Both girls had been drinking before the attack and the older girl told the court she had taken prescription drugs earlier in the day.
Ms Wrightson went to the shop for the girls on the night she was killed (BBC) Source: BBC
The pair phoned police following the attack and asked for a lift home before taking a selfie on a mobile phone in the back of the police van.
The court also heard that the pair sent photos of the victim's bloodied face to friends using the Snapchat social media app.
Gerry Wareham, of the Crown Prosecution Service, said: "In our society it is hard to imagine that two girls of such a young age could be capable of such violence."
"The attack that the girls committed against Angela Wrightson was brutal and sustained. One can only imagine the fear and distress that she must have felt in the final hours of her life," Mr Wareham said.
One of the girls took this photo in the police van (BBC) Source: BBC
"Given the severity of their assault on Ms Wrightson, one would expect the girls to have shown a degree of remorse in the wake of her death.
"Instead, they laughed and smiled while posing for a selfie, with each continuing to deny that they had murdered her throughout the investigation and prosecution of this case."
A statement from the victim’s family after the verdicts said: "Angie was attacked and brutally murdered in her own home, a place where we all have the right to feel safe.
This image, which was posted on Snapchat by one of the girls, shows Ms Wrightson with marks on her face (BBC) Source: BBC
"Listening to the details of her injuries and of her final moments has been a harrowing experience and something which will continue to haunt us each and every day.” the statement read.
"No sentence, regardless of its severity, will ever bring Angie back."
Hartlepool Borough Council confirmed with the BBC that independent Serious Case Reviews are being carried out in respect of the two children.
Both girls were in tears upon conviction and they will be sentenced on Thursday.
