Poor families give their children to people who promise they'll have a better life in Britain and other European countries.
But the children often end up on sale in local markets.
A BBC reporter in the Ivory Coast was offered a 14-year-old girl to buy for about £15.
In Britain, the children can be used as domestic slaves. Sometimes they're beaten, starved or sexually mistreated.
Last year, eight-year-old Victoria Climbie from the Ivory Coast died after her aunt and aunt's boyfriend beat and abused her.
Her parents thought she would have a better life in Britain.
They didn't realise that their dreams had become a nightmare.e.