There are 5,000 prostitutes under the age of 18 in the UK, according to a charity report.
Save the Children has highlighted eight forms of child slavery
Save the Children's figures suggest three-quarters of these are girls, and 1.8 million children are being abused in the sex industry across the world.
The report, marking 200 years since the end of the British slave trade, says 1.2 million children and babies are trafficked every year worldwide.
The gangs involved make an annual £15bn profit, the charity says.
Save the Children's head of protection Bill Bell said: "Child slavery is not a thing of the past.
"There are still millions of children in both rich and poor countries who are being forced to lead slave-like lives under horrific conditions of humiliation and abuse.
"These children are treated as commodities, liable to be lent or sold to other owners without warning."
Save the Children says the eight most common forms of child slavery today are:
child traffickingchild prostitutionbonded child labour (to pay off debt)forced work in minesagricultural labourchild soldiersforced child marriagedomestic slavery
The report states one million children are risking their lives in mines and quarries in more than 50 African, Asian and South American countries.
About 132 million children under the age of 15 are working in agriculture, often exposed to pesticides, heavy machinery, machetes and axes, it says.
There are 300,000 children involved with fighting forces, including government armies, the report says.
Child marriage - including mail order and internet brides - is one of the most widespread forms of slavery, it adds.
The charity called on the world's governments to ensure an end to child slavery.
A Home Office spokesman said: "The government is committed to ensuring the protection of children, and putting an end to human trafficking."
The spokesman said the UK's Action Plan on Tackling Human Trafficking, published earlier this year, contained a "comprehensive strategy to improve the identification and safeguarding of child victims of trafficking".