An eight-year-old was among more than 22,000 children held overnight in police cells last year, according to new figures.
Another one of the 22,792 under-18s was held for 15 days, data obtained by the BBC through a Freedom of Information request showed.
The eight-year-old was held overnight by Gloucestershire Police.
But the 2014-15 figures, from 39 forces in England, amounted to almost half the total for 2011-12, when 41,789 were held in police custody overnight.
Holding children in police cells is expected to be an emergency measure when other secure accommodation is unavailable and once charged a youth suspect should be either bailed or transferred to council accommodation, unless it is impracticable, such as moving them in the middle of the night.
Merseyside Police said just three out of 73 children were transferred to council accommodation in June and July 2015.
Forces told the BBC there was a "lack of alternative accommodation".
But Frances Crook, chief executive of the Howard League for Penal Reform, told the BBC: "The police know cells are not a nursery or a school. They are not an appropriate place for children to be.
"Police stations are noisy and full of adults - some of whom are drunk and dangerous.
"The cells are often subterranean and really unpleasant places."
