The latest figures from the UN refugee agency show an increase of 60% last year, with more than 6,000 under-18s seeking asylum.
Unicef says there is an urgent need to protect children migrating alone.
The UN Convention on the Rights of the Child obliges countries to protect children of all nationalities.
In 2008, UN refugee agency figures show, 3,800 Afghans under 18 applied for asylum in Europe.
The UN believes the latest figures are the tip of the iceberg; many unaccompanied minors do not apply for asylum because they fear detention and deportation.
A new study from Unicef, the UN children's agency, reveals a lack of coherent policy within Europe towards child migrants and often a failure to protect them.
Aid workers point to a recent case in which two Afghan boys, one only 13, died trying to hide in lorries travelling from Greece to Italy.
A study of care offered by UK local authorities shows, Unicef says, that unaccompanied children often experience racism, are not believed when they try to tell adults their stories, and struggle with the mental trauma experienced on their journey to the UK.
The UN says much more research into the growing phenomenon of child migrants is needed, but that these initial findings show the need to offer better protection is urgent.
BBC News
