MARYNIVKA, Ukraine—Alexander Rybalko climbed down the brown, sloping field behind his blown-up dacha and pointed to a roughly three-foot-wide crater in the dirt.

It was here, on Sept. 3, that he was out digging potatoes with his 17-year-old son, Dmitry, when the teenager noticed a strange object in the soil and threw a potato in its direction. The object detonated in his son’s face.

He cradled Dmitry in his arms, carried him to the car and rushed him to a nearby hospital, but it was too late. His son died—the victim of what Mr. Rybalko said was a latent bomblet seeded weeks earlier into his garden by a cluster munition that was fired during fighting over the village.

“I want to find that rocket. It is probably somewhere over there,” Mr. Rybalko said, pointing toward the neighbor’s garden. So far he says it is too dangerous to look.

Separatist-held areas of eastern Ukraine such as Marynivka, which once formed the front line in the conflict between Ukrainian forces and Russia-backed rebels, remain littered with unexploded ordnance, posing a lurking danger to residents already struggling to pick up the pieces of their lives.