A top lawyer and Liberal Democrat peer yesterday revealed he had been frightened by techniques used to subdue child prisoners.
Lord Carlile of Berriew said he asked custody staff to use "control and restraint" methods on him as part of an independent inquiry, sparked by the death of a teenager at a Warwickshire institution, into their use on offenders under the age of 18.
He said: "I found the techniques ranged between the broadly acceptable and the frankly unacceptable.
"The most unacceptable was not a technique which involved pain - it involved three members of staff placing themselves in martial arts challenging poses before me.
"The way in which they positioned themselves was supposed to de-escalate the situation and it made me a little bit frightened - and I don't frighten easily."
The report, commissioned by the Howard League for Penal Reform, followed the death of 15 year-old Gareth Myatt.
The teenager weighed just seven stone when he died in April 2004 after being restrained by three members of staff at the privately run Rainsbrook secure training centre near Rugby.
Lord Carlile, a former MP who is also the Government's independent anti-terror watchdog, said responsibility for under-18s in custody should be taken away from the Prisons Minister and handed to the Children's Minister.
He described how staff demonstrated three different restraint techniques on him, including a so-called distraction technique involving an upward jab to the nose.
None of the demonstrations drew blood, he added.
"I felt that I could not conduct this exercise without having the techniques administered on myself.
"I was a willing victim - I wasn't 'kicking off'.
"The response to pain, especially when one is angry, is for one to become agitated and that is in the background, I believe, of some of the tragic events, including the death of Gareth Myatt, which triggered this report."
Lord Carlile, who made 45 recommendations in all, said routine strip-searching of children in prison should end and called for the use of physical force to be severely restricted.
He said he had been "shocked" by the treatment of child offenders.
A child abuse inquiry would normally be triggered if children elsewhere were treated in the same way, he added.
The 100-page report discloses that some staff would "bait" children into situations that would lead them to being restrained for the adult's sexual gratification, and that some young detainees would deliberately seek physical force to gratify their own sexual needs.
The peer also said he was "appalled" there were no playing fields or outdoor facilities at any of the institutions the inspection team visited.
"The lack of exercise and daylight would seem to me to contribute to depression and conflict amongst adolescents," he said.
Earlier this month, the Youth Justice Board, which oversees child custody, launched a new programme to reduce the use of force in child prisons. It unveiled a new code setting out when physical restraint can be used.
YJB chairman Professor Rod Morgan said: "Over the past few months, we have made significant improvements in these areas and I am delighted that much of what this report calls for is already in place."
Shadow Prisons Minister Edward Garnier welcomed the report's call for prison reform but added: "We need a report that shows how we can make prisoners accept their responsibilities."
Liberal Democrat home affairs spokeswoman Lynne Featherstone said: "The report highlights why many children currently in secure accommodation would be better undergoing rehabilitation outside of what all too often amount to colleges of crime."