The UN Office of the High Commissioner for Human Rights (OHCHR) said its conclusions were based on a visit to the site a month after the attack.
At the time, human rights groups blamed the deaths on an Israeli air strike.
The Israeli military says it never denied carrying out the strike because it was not clear what had happened.
The UN says 33 other Palestinian children died in Israeli attacks during the conflict.
A photo of BBC video editor Jehad Mashhrawi cradling the corpse of his baby son Omar became one of the iconic images of November’s short war.
Omar was killed, along with an aunt and an uncle, after a missile hit the family home in Gaza City.
It happened only an hour after Israel launched its operation with the killing of Hamas’s military commander.
The family, and human rights groups, said that the house was hit in an Israeli attack.
The Israeli military made no comment at the time of the incident but never denied carrying out the strike.
Privately, military officials briefed journalists that they had been targeting a militant who was in the building.
Now, though, the United Nations says the house may have been hit by a Palestinian rocket that fell short.
This is despite the fact that the Israeli military had reported no rockets being fired out of Gaza so soon after the start of the conflict.
UN officials visited the house four weeks after the strike.
They said they did not carry out a forensic investigation, but said their team did not think the damage was consistent with an Israeli air strike.
However, the UN said it could not “unequivocally conclude” it was a misfired Palestinian rocket.
A UN official said it was also possible the house was hit by a secondary explosion after an Israeli air strike on Palestinian weapons stores.
Jehad Mashhrawi dismissed the UN findings as “rubbish”.
He said nobody from the United Nations had spoken to him, and said Palestinian militant groups would usually apologise to the family if they had been responsible.
An Israeli military spokesman said he could not comment on the accuracy of the UN’s findings but said it would not be the first time a Palestinian rocket had misfired.
He said that, in the intense first hours of the conflict, it was not always clear what was happening.
The UN report concluded that at least 169 Palestinians were killed by Israeli attacks during the offensive.
It said more than 100 were civilians, including 33 children and 13 women. The report said six Israelis were killed by Palestinians attacks, including four civilians.
Reprinted from BBCnews