The charity, Save the Children, has collected harrowing first hand accounts from Syrian children, who say they've been tortured and witnessed atrocities.
The charity says almost every child it's spoken to who has managed to get out of Syria has talked of having seen a family member killed.
It says children in Syria are subjected to "appalling" torture, imprisonment and abduction, and these atrocities should be better documented.
The charity is calling on the United Nations to increase its presence on the ground.
The report comes at the start of the United Nations General Assembly's annual meeting.
Earlier, UN envoy Lakhdar Brahimi said the situation in Syria was "extremely bad and getting worse".
Speaking after he had briefed the Security Council following his first visit to Syria since taking up the post, Mr Brahimi admitted he did not have a full plan on how to bring peace.