Scotland's chief inspector of prisons has expressed concern at the number of children who have been in jail in the past 12 months.
Children are still being detained at Polmont, the report says
Children were being held in a young offenders' institution more than a year after inspectors advised against it.
The Polmont Young Offenders' Institute near Falkirk is designed to hold 16 to 21-year-olds, but 11 children have been held there in the past year.
Dr Andrew McLellan's report said the practice breaches UN guidelines.
Four of the 11 children held at Polmont were deemed as "too difficult" to be contained at secure units, the report said.
The Scottish Executive has said it is providing more secure accommodation places for children.
But charities said too many were being taken by youngsters who could be looked after by intensive fostering in the community.
It is understood one child, who was held for 130 days, was ordered to Polmont by a sheriff concerned that his behaviour was a danger to himself and others.
The study, carried out in March this year, suggests that the prison is breaching children's rights as defined by the UN, which states that they should be imprisoned as "a last resort".
"Children under the age of 16 are still being held in Polmont," the report said.
Dr Andrew McLellan: Concerned at the number of children behind bars
"The UN Convention on the Rights of the Child states that 'In all actions concerning children... the best interests of the child shall be a primary consideration'."
The report also said the convention states that imprisonment of a child 'shall be used only as a measure of last resort and for the shortest appropriate period of time'.
Dr McLellan's report also criticised the fact that "slopping out" remains a problem to be overcome at Polmont, despite improvements having been made to the arrangements.
It said: "Slopping out is always bad - no amount of improvement can make it good.
"It is particularly bad that it happens in a hall which holds prisoners on remand.
"A good feature of most Scottish prisons is that prisoners who have not been convicted usually live in good conditions.
"It is very upsetting that the only remand prisoners who are regularly slopping out are under 21 years of age."
Nevis Hall, last year described as "tense and oppressive", was found to have a relaxed atmosphere and is now the focus of a drugs addiction programme.
The report went on: "The very bad conditions in Nevis Hall identified in last year's report have been addressed."
Scottish National Party justice spokesman, Stewart Stevenson, said: "It is totally unacceptable that children under the age of 16 are being held at the Polmont Young Offenders Institute.
"The best interests of these children must be the executive's primary consideration, and action must be taken immediately to stop children being locked up."