The UN has added two groups in Yemen and two in Iraq to its annual list of those recruiting or abusing children during armed conflicts.
During Yemen's recent civil war, as many as 15% of the pro-government militia and 20% of the Huthi rebels were made up of children, the UN says.
Even though that conflict was now over, nobody knew where the children were, said a UN special representative.
The annual report said children were involved in warfare in 22 countries.
"2010 proved another tragic year for children in conflicts all over the world," said Radhika Coomaraswamy, Special Representative of the Secretary General for Children and Armed Conflict.
"We've taken no parties off of the list and added four more."
She also raised concerns about the reference to attacks on schools and hospitals in 15 of the 22 countries listed.
As well as physical attacks, some schools and hospitals had also been forced to close due to direct threats, intimidation, or military occupation.
"Schools must be safe places of learning and development for all children," said Ms Coomaraswamy. "They should be zones of peace. Those who attack schools and hospitals should know that they will be held accountable."
UN Secretary General Ban Ki-moon added his concern over the increase in attacks on schools and hospitals, noting that schools were commonly used as recruiting centres for child soldiers.
In Sudan alone, 15 groups were cited for recruiting and using, killing and maiming, or committing rape and other forms of sexual violence against children during wartime.
The report also detailed violations against children in Afghanistan, Burma, Burundi, Central African Republic, Chad, Congo, Colombia, Haiti, India, Iraq, Ivory Coast, Lebanon, Nepal, Israel and the occupied Palestinian territories, Pakistan, Philippines, Somalia, Sri Lanka, Thailand, Uganda and Yemen.