A boy at a young offenders institution was left to lie on a mattress on the floor of a "filthy" cell for more than 22 hours a day, a report has revealed.
The chief inspector of prisons in England and Wales, Peter Clarke, said a practice of separating children from their peers at YOIs amounted to "harmful solitary confinement".
He said the policy had "fundamental flaws" and was a risk to mental health.
The government said it would be making "immediate changes".
Inspectors looked at five YOIs, holding about 600 men and boys aged 15 to 18 - Cookham Wood in Kent; Feltham A in west London; Parc in south Wales; Werrington in Staffordshire, and Wetherby and Keppel in West Yorkshire.
Mr Clarke said 57 offenders had been separated and "in the worst cases children left their cells for just 15 minutes a day".
His report, after inspections in May and June 2019, found "multiple and widespread failings", although some areas of better practice were identified, particularly at Parc.
It said there were occasions when it was in a child's best interests to be separated for the risk they posed to others, or for their own protection. But staff should still aim to ensure they have daily activities and work to reintroduce a normal regime.
'Worse than an adult prison'
Sheena Evelyn says her son, Kyefer, spent "most of his time" in solitary confinement when he was in a young offenders institution.
She first spoke to the BBC in 2018 to support calls for reform of the practice which she called "cruel and inhumane".
"In a short space of time he was hearing voices, he was suicidal, just a massive impact overall on his mental health at the time but in the long-haul we still don't know how much it has affected him," she told BBC Radio 4's Today programme on Tuesday.
"It's normal common sense, it doesn't take a doctor to tell you that a child needs basic rights like sunlight and interaction with their peers."
Kyefer is now in an adult prison, where Ms Evelyn says he is "treated more humanely" than in Feltham YOI.
"He gets basic exercise, he gets sunshine, he gets adequate amounts of food - whereas these growing children are given miserly portions. They're given snack-sized breakfasts given the evening before so that most of them eat them that evening and they're starving the next day."
Asked why her son was put into solitary confinement she says "there doesn't have to be a reason".
She called its use an "abuse of power".
Ms Evelyn adds: "They're very violent places young offenders (institutions), very violent. There's a lot of testosterone, they're locked behind the doors for 23-and-a-half hours a day.
"You have to fight to survive in these places."
Mr Clarke found eight children had spent a combined total of 373 days in separation while waiting to be taken to a secure hospital for treatment for mental health conditions.
He said nearly all those separated spent long periods in cells "without any meaningful human interaction".
This included the case of the boy left on the mattress, which took place at Feltham A, where he had been "in crisis" and on a "constant watch".
Some were "unable to access the very basics of everyday life, including a daily shower and telephone call", Mr Clarke added.
On Tuesday, he told the Today programme, that the system was "badly broken" and needed to be designed "afresh".
"These failings are so consistent and have been taking place for such a long time, I've taken the rather unusual step of putting one overarching recommendation into our report which is that the secretary of state for justice should take personal charge of this and actually insist that something is now done to put this right," Mr Clarke said.
"We don't deny for a moment there are occasions when children do need to be separated but if they do need to be separated - and that's very often for their own or somebody else's safety or for other good reason - then it should be done in a way which allows them to have the basic entitlements, and that hasn't been happening."
Justice minister Wendy Morton said: "It is difficult to read this report and not conclude that we are failing some of the children in our care. That is completely unacceptable and I am determined it will not continue."
She said separation can be "necessary" but there was "no excuse for some of the practices highlighted in this report and I have asked my officials to urgently set out the steps we need to take to stop them happening".