Pillay noted an Israeli drone missile strike in Gaza City that killed three children and wounded two others while they were playing on the roof of their home. She also referenced an Israeli strike and naval shelling that struck seven children playing on Gaza beach, killing four from the same family.
"These are just a few examples where there seems to be a strong possibility that international humanitarian law has been violated, in a manner that could amount to war crimes," Pillay told the 47-nation U.N. Human Rights Council.
Israel launched its operation in Gaza on July 8 in response to heavy rocket fire out of Hamas-controlled Gaza. The fighting escalated last week with an Israeli ground offensive.
Pillay also warned that Hamas and others were violating international law.
"Israeli children, and their parents and other civilians, also have a right to live without the constant fear that a rocket fired from Gaza may land on their houses or their schools, killing or injuring them," Pillay said.
"Once again, the principles of distinction and precaution are clearly not being observed during such indiscriminate attacks on civilian areas by Hamas and other armed Palestinian groups," she added.
Pillay said not abiding by those principles could amount to war crimes and crimes against humanity.
At the session, Palestinian Foreign Minister Riad Malki accused Israel of committing a crime against humanity and violating international human rights law. Israeli Ambassador Eviatar Manor accused Hamas of committing war crimes.