Hundreds of millions of children suffer discrimination and exploitation but are invisible to the world, the UN agency for children says in its annual report.
One in 13 children in the developing world have lost a parent, it said
They include trafficked children - some of whom are sold for sex - who "disappear" from mainstream society.
Others, including street children, are denied basic rights such as schooling and healthcare.
Children not registered at birth are among those most likely to be forgotten and invisible, the report said.
It said new laws are required to ensure that births are officially registered, and it also urged governments to do more to stop "abuse and exploitation".
The report said nearly two million children had entered the sex trade, 5.7 million were sold into slavery and 1.2 million were trafficked each year.
"These numbers are huge, and we do have to push several buttons in every case," said Unicef child protection chief Karin Landgren. "So we have to start by shining that light on the plight of these children."
The State of the World's Children 2006: Excluded and Invisible said exploited children were often overlooked in public debate or news stories.
WHY CHILDREN BECOME INVISIBLE
More than 50 million children a year are not registered
One in 13 children in the developing world has lost a parent
Children forced into adult roles miss out on vital stage of development
Exploited children include more than two million in the sex trade
Source: The State of the World's Children 2006: Excluded and Invisible
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"Meeting the Millennium Development Goals depends on reaching vulnerable children throughout the developing world," said Unicef executive director Ann M. Veneman.
"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."
The report said over half of the births in the developing world - apart from China - are not registered, meaning they are not recognised as citizens.
Without registered identity, they are unable to receive education, decent health care and other services. If they do not officially exist, it also means traffickers can make them "disappear" without fear of retribution.
"Trafficking needs to be looked at as a global problem that is not just a developing world problem... because the demand often comes from the developed world," Ms Veneman said.
"The recommendations in this year's report particularly make it clear that it is not just governments that are involved here. Civil society has a huge role to play, communities have a huge role to play," the report's author David Anthony said.
"It takes bold and courageous action to tackle some of these things in many countries," he added.