The report, released Tuesday, indicated armed militant groups -- including the Islamic State of Iraq and Syria and Jhabat al-Nusra -- use children to handle ammunition, operate checkpoints, participate in attacks on schools, and sometimes as combatants. The children are subject to death, injury and sexual violence.
The most critical examples are in Syria, Iraq, Nigeria and South Sudan, the report said.
The U.N. estimated over 10,000 children were killed in fighting in Syria in 2013.
On the topic of South Sudan, report author Leila Zerrougui, U.N. envoy on children and armed conflict, said, "I saw (for) myself children with guns; with the government, but also with armed groups -- with David Yau Yau, with the SSLA (Southern Sudan Legislative Assembly) and with the opposition under Riek Machar."