Some children said they had been forced to ride on tanks to stop attacks by opposition fighters, the report said.
She said she had never seen a similar situation where children were not spared - and even targeted - in a conflict.
"Many former soldiers spoke about shooting into civilian areas, seeing children, young children being killed, and maimed," she said.
"We also had testimonies and saw children who had been tortured, and who carried the torture marks with them. We also heard of children being used - this was recounted to us by some children - of being put on tanks and being used as human shields so that the tanks would not be fired upon."
However, she also criticized the opposition Free Syrian Army for endangering children.
"For the first time we heard of children being recruited by the Free Syrian Army mainly in medical and service orientated jobs but still on the front line," she said.
Ms Coomaraswamy said the suffering inflicted on children in Syria was unusual even for combat situations.
"We are really quite shocked. Killing and maiming of children in cross-fire is something we come across in many conflicts but this torture of children in detention, children as young as 10, is something quite extraordinary, which we don't really see in other places."
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