One in three victims of human trafficking worldwide is a child, many of which are subject to sexual exploitation or forced labour, according to a UN report from the Drugs and Crime office.
Overall, child trafficking has increased 5% since 2010, with girls and women accounting for 70% of the overall number of victims worldwide.
"There is no place in the world where children, women and men are safe from human trafficking," said UNODC Executive Director Yury Fedotov.
The UN said the global figures represented only "the tip of the iceberg" and that impunity remained a serious problem.
"It is very clear that the scale of modern-day slavery is far worse," said Mr Fedotov.
Women and girls are often trafficked for sexual exploitation and forced labour, while children are also forced into combat or to take part in petty crime, the report found.
In 2003-2006, about 20% of human trafficking victims were children, the report found, indicating how much the problem has increased over the past few years.
Children alone represent around 60% of victims in regions such as Africa and the Middle East.
The report also highlighted that the number of convictions remain low despite initiatives to combat trafficking.
"40% of countries recorded few or no convictions, and over the past 10 years there has been no discernible increase in the global criminal justice response to this crime, leaving a significant portion of the population vulnerable to offenders," the UN said.
About 15% of the 128 countries covered by the report did not record a single conviction, it said.