One nine-year-old boy said his father had been shot dead in front of him despite surrendering to Israeli soldiers with his hands in the air.
Another youngster described witnessing the deaths of his mother, three brothers and uncle after the house they were in was shelled.
He said his mother and one of his siblings had been killed instantly, while the others bled to death over a period of days.
A psychiatrist treating children in the village of Zeitoun on the outskirts of Gaza City, where the alleged incidents took place, described the deaths as a "massacre".
Rawya Borno, a Jordanian doctor, said civilians, including children, were rounded up and killed by Israeli troops.