Some 8.4 million Syrian children have been affected by the conflict that has dragged on since 2011, UNICEF said in a report Monday.
About one in three of them - 3.7 million - have been born since the beginning of the war.
The UN agency said that during the past five years, children's lives have been shaped by fear, violence, hunger and displacement. An estimated 2.4 million children have had to flee to other countries, and 306,000 children were born as refugees.
"We cannot restore the precious years of childhood snatched away by this brutal war, but we can and must prevent their futures from also being stolen," said Anthony Lake, UNICEF executive director, in the report.
UNICEF said that 2 million Syrian children have no regular access to humanitarian aid and more than 200,000 children live in besieged areas.
The UN has also verified almost 1,500 cases of grave violations - such as killings or maimings through the use of explosives - against children in 2015.
The report warned that while the recruitment of child soldiers aged 15 to 17 had been reported before, children as young as 7 are now also being used by parties to the conflict.
"As the war continues, children are fighting an adult war," said Peter Salama, UNICEF's regional director for the Middle East and North Africa. "They are continuing to drop out of school and many are forced into labour, while girls are marrying early."
UNICEF called for an end to violations against children, the lifting of sieges and securing the 1.6 billion dollars in funds requested by UNICEF for providing aid and education to Syrian children.