Growing numbers of lone migrant children arriving at Dover have led the government to call on councils outside Kent to “take responsibility” for them.
More than 200 places have been pledged by 48 authorities across England.
Children are waiting at the Kent Intake Unit, a Border Force facility in Dover, before being placed in social care.
Campaigners said keeping children there was worrying, but the Home Office said children were being prioritised and stayed there as briefly as possible.
‘Entirely preventable situation’
The move to keep asylum-seeking children at the facility was announced this week after Kent County Council said it could not safely care for any more child migrants.
But Bella Sankey, director of Detention Action campaign group, said the situation was a “scandal”, adding: “It is deeply worrying that this entirely preventable situation has occurred.”
Kent Refugee Action Network said Border Force officers did not have the skills to care for vulnerable children.
Officials have said washing facilities, food and places to sleep are provided at the Dover unit and there is a separate room for families and lone children.
By law, children can only be detained for 24 hours.
More than 400 children, most of whom arrived in Dover across the English Channel by small boat, have entered Kent County Council’s care so far this year.
The BBC has asked the Home Office how many children are currently at the Dover unit, and how long they have stayed there for.
On Thursday, the government said the Home Office, Department for Education and Ministry of Housing, Communities and Local Government were writing to local authorities “urging them to come forward, play their part and take responsibility”.
A Home Office spokeswoman said: “We continue to provide Kent County Council with support, including significant increases in funding, but the burden being placed on them is unacceptable and cannot continue.
“We are grateful to the 48 local authorities who have pledged more than 200 places to support our National Transfer Scheme, but we need more to come forward and do their bit for vulnerable children.”
Critics have said the voluntary National Transfer Scheme should be mandatory for councils and the government said the scheme would be reviewed.