On Tuesday I went to visit a group of refugee children in police custody in a village near Idomeni, on the Greek border with Macedonia. The policeman banged open the lock of the black metal cell door and it swung forward. The other boys moved aside to let Harith through. The door clanged shut. Harith and the seven boys with him are refugees from Syria, Iraq and Pakistan; they are all aged between 14 and 17. Under Greek law, unaccompanied minors are supposed to be held in police custody only until they can be transferred to centres for young people. But, all over Greece, the centres are full. Harith has been in jail for more than two weeks.