In a major report exploring the causes of exclusion and the abuses that children face, UNICEF said millions of children disappear from view when trafficked or forced to work in domestic servitude.
Other children, such as those living on the street, remain visible but are often left out of fundamental services and protections, including schooling and health care, said the report.
The State of the World's Children 2006L Excluded and Invisible, said exploited children are often missing from public debate, official statistics and news coverage.
"To ensure that children are protected, the abuse and exploitation of children must be brought to light and those who violate children brought to justice," said Ann M Veneman, UNICEF's executive director, releasing the report in London.
"There cannot be lasting progress if we continue to overlook the children most in need -- the poorest and most vulnerable, the exploited and the abused," she said.
The report calls for a greater focus on these children, particularly in terms of meeting the UN Millennium Development Goal of universal primary education.
Children at risk include those without a formal identify, with more than half of all births in developing countries, excluding China, not registered, denying them recognition as citizens and compromising their access to education, healthcare and other services.
Children without parental care are also highlighted, as well as street children and more than one million in detention worldwide.
The report also points out that around 171 million children work in hazardous conditions, including factories and mines, and hundreds of thousands work in armed conflict, as well as children who are exploited in child labour, including debt bondage, prostitution and domestic servitude.
UNICEF appealed for "a massive push" to boost access to essential services for children and their families, starting with "quick-impact initiatives" that can kick-start development and reduce poverty.
Longer term, it proposed a stepping-up of initiatives "rooted in a human rights-based approach to development" to ensure that quick-impact policies lead to sustainable results.