Rebels have also recruited youngsters as soldiers and used terror tactics in civilian areas, according to the UN's first report on this area of the conflict that was released to the Security Council on Tuesday.
It detailed gruesome incidents of torture including children being raped or beaten with metal cables, suffering electric shocks to the genitals and having their fingernails ripped out.
"Violations must come to an end now," UN Secretary General Ban Ki-moon said in the report.
The report covered the period from March 1, 2011 to November 15 last year in a conflict that has now left more than 135,000 people dead.
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It details a series of abuses Syrian children have suffered since the opposition rose up to try to depose President Bashar al-Assad.
They range from direct commission of abuse, including sexual violence, to more general violation of their rights, from school closures and denial of access to humanitarian aid, the UN website said, quoting from the report.
"Government forces have also been responsible for the arrest, arbitrary detention, ill treatment and torture of children," he added.
"Armed opposition groups have been responsible for the recruitment and use of children both in combat and support roles, as well as for conducting military operations, including using terror tactics, in civilian-populated areas, leading to civilian casualties, including children.