Al Jazeera's Sherine Tadros in Gaza, where more than half of the population of 1.5 million people is under 18-years of age, said Sarraj told the panel that six months after the war the trauma is still present among children.
"During the war we spent the night with a family and we saw first hand the kind of trauma that Dr Sarraj was talking about in terms of the children and how frightened they were when the bombs were going off," she said.
The panel also heard from the head of a women's group in Gaza City, who said that most of those who died in the conflict were men, leaving behind the women they provided a livelihood for, Tadros said.
"Even months after the war the women are still suffering because they have lost their livelihood and have to go out and work," she said, adding that this was flagged up as a "major problem".
The hearing, which is being broadcast live for the public, will also include testimony from experts on the military operation on the Palestinian enclave.
The panel is to hold a second round of public hearings on July 6 and 7 in Geneva where it will hear from the victims of alleged violations in Israel and the West Bank.
The UN chose the Swiss city as the venue of the second round of hearings because the fact-finding mission did not receive permission to enter Israel.
The public hearings were called for by Richard Goldstone, the head of the 15-member team and previously a member of the South African constitutional court.
The mission is due to complete a report with its findings in August.
Israel launched its offensive against Gaza on December 27, citing rocket attacks from the enclave that caused injuries to residents and damage to property in Sderot and other towns.
The military operation killed more than 1,400 Palestinians, including more than 900 civilians, among them scores of children, according to Palestinian officials and human rights groups.
It also destroyed thousands of homes and heavily damaged Gaza's infrastructure.
Israel says the death toll was lower and most of the dead were Hamas fighters.
Thirteen Israelis were also killed during the fighting.
Gaza's reconstruction is being hampered by Israel's blockade of Gaza, which dates back to June 2007 when Hamas took control of the territory.
Since then, Israel and Egypt, which control Gaza's only border crossing that bypasses Israel, have kept the territory of 1.5 million aid-dependent people sealed to all but essential humanitarian supplies.
Israel has insisted that the blockade is necessary to prevent Hamas from arming itself. Human-rights groups say it is a "collective punishment" that wrongly hurts ordinary civilians.
The fact-finding mission is mandated by the UN to investigate all violations of international human rights and humanitarian laws that might have been committed at any time in the context of the military operations conducted in Gaza.