Beirut: Khalid, 15, said he was hung by his arms from the ceiling of his own school building in Syria and beaten senseless. Wael said he saw a 6-year-old starved and beaten to death, "tortured more than anyone else in the room".
The first-person accounts come from interviews with refugees who have fled the Syrian conflict conducted by the British-based charity Save the Children and published on Tuesday.
The report did not say who had abused the children, but a spokesman for Save the Children said some had heard their parents blaming government forces for the attacks.
UN investigators say Syrian government forces have committed human rights violations "on an alarming scale", but have also listed multiple killings and kidnappings by armed rebels trying to topple President Bashar al-Assad.
The children that Save the Children spoke to in refugee camps in neighbouring countries said they had witnessed massacres and seen family members killed during the 18-month-old conflict.
"I knew a boy called Ala'a. He was only 6 years old. He didn't understand what was happening. His dad was told that this child would die unless he gave himself up," said Wael, 16, who like all the children interviewed was not identified by his full name or location.
"I'd say that 6-year-old boy was tortured more than anyone else in the room. He wasn't given food or water for three days, and he was so weak he used to faint all the time," Wael was quoted as saying. "He was beaten regularly. I watched him die. He only survived for three days and then he simply died."
Opposition activists say 27,000 people, mostly civilians, have been killed in Syria's bloodshed. Many of the civilians died initially in attacks by security forces on peaceful protests. Others have been killed in government shelling or in crossfire during the ensuing civil war.
Khalid, 15, said he had been taken along with over a hundred others to his old school, which had been turned into a torture centre, and had his hands tied with plastic cord.
"They hung me up from the ceiling by my wrists, with my feet off the ground, then I was beaten. They wanted us to speak, to confess to something," he said.
"I passed out from the severe pain of hanging like that, and from the beating. They took me down and threw cold water on my face to wake me up. Then they took turns stubbing out their cigarettes on me. Here, I have these scars."
Omar, 11, described life under bombardment.
"One day I was playing with my brothers and my cousin. We were teasing her and she was upset. She left us and went to her house. That night, a shell destroyed my 9-year-old cousin's house - the one we'd upset during the day. I regret that she died feeling sad," he said.
Another interviewee, Munther, 11, said that he and several other children were standing outside his school when bullets started whizzing by.
"A boy called Amjad was standing next to me. He was shot in the head. I didn't realise at first that he was dead. He fell forward on his knees, in a praying position," Munther said.
"Then I felt a terrible pain. I'd been shot too - in my neck," he added, pointing to two scars.
Save the Children chief executive Justin Forsyth, who heard the reports first-hand, said the stories "need to be heard and documented so those responsible for these appalling crimes against children can be held to account".
The charity urged the United Nations to increase its presence on the ground to enable it to document every crime.