According to Navi Pillay, hundreds of children were being held, tortured and interrogated in prisons without medical care.
"They've gone for the children - for whatever purposes - in large numbers. Hundreds detained and tortured... it's just horrendous," she said.
"Children shot in the knees; held together with adults in really inhumane conditions; denied medical treatment for their injuries; either held as hostages or as sources of information," she said.
Pillay said that the atrocities could be brought to an end on the President's order, still nothing was done.
"That is the legal situation. Factually, there is enough evidence pointing to the fact that many of these acts are committed by the security forces [and] must have received the approval or the complicity at the highest level, because President Assad could simply issue an order to stop the killings and the killings would stop," she said.
Radhika Coomaraswamy, the UN special representative for children had said that the Free Syrian Army troops were allegedly using child soldiers.