The boy assumed that Layla’s mother was already on the bus. In fact, she was in another part of the house, suffering from a toothache. Still, Layla was hardly the only infant at the protest. Entire families had come along, some snacking on ice cream or sandwiches, as the protests raged hundreds of yards away.

In the late afternoon, Layla, in a tent with her aunts, started to wail. Ammar grabbed his niece for a second time and, he said, pushed forward into the protest in search of her grandmother, Heyam Omar, who was standing in a crowd under a pall of black smoke, shouting at Israeli soldiers across the fence.

Soon after Ms. Omar took the child, she said, a tear-gas canister fell nearby. She frantically wiped the child’s face with water and gave her juice to drink. But an hour later, after they reached the family home, Layla appeared to have stopped breathing.

When they arrived at a hospital at 6:34, doctors pronounced the child dead. “Her limbs were cold and blue,” reads a hospital report.

Layla’s mother crumpled onto the hospital bed and wept over her daughter.

“I felt like my heart had been attacked,” she said.

The rules of grief in Gaza, where private pain is often paraded for political causes, kicked in. The next morning the secular Fatah movement erected a funeral tent outside the family’s home, and hung a banner with a photo of the infant beside an image of Mahmoud Abbas, president of the Palestinian Authority.

It was not the first public death in the family.

A large poster in the family living room shows Mariam Ghandour’s uncle, Ammar, brandishing a rifle and wearing a black headband. A member of Al Aqsa Martyrs Brigades, a militant group affiliated with Fatah, he died battling Israeli soldiers in 2006.