Unicef is appealing to the British government to speed up the transfer of unaccompanied child refugees from the "Jungle" migrant camp in Calais.
The charity says it is concerned the planned closure of the camp will lead to children there disappearing before they are processed.
Charities estimate there are about 400 unaccompanied children in the Jungle eligible to come to Britain.
The Home Office says it is committed to resettling "vulnerable children".
On a visit to Calais on Monday, French President Francois Hollande pledged to clear the Jungle camp by the end of the year.
The thousands of adults and children there will be relocated to reception centres.
Unicef fears that children will resist being sent to such centres while their cases are being processed.
Lily Caprani, Unicef UK's deputy executive director, told the BBC: "Last time part of the Jungle camp was demolished, hundreds of children went missing. We don't know what happened to them."
She added the charity fears they could fall into the hands or traffickers or those who would exploit them.