For anyone who doesn't want to read the article. The children are people who's age has been disputed and the Home Office (Border Control) has declared them adults.
An organization working with age-disputed people says they identified 14 children that have been sent to an adult prison. The youngest is believed to be 14 years old. In total there are well over 1000 age disputed people.
Doesn't make the situation much better but explains how it happened. Also important, sometimes the only crime committed was seeking asylum. And officially sex offenders (and similar criminals) aren't supposed to be at that facility anymore. But they still are.