GARDENDALE, Tex. — Kris loved to soar on the swings. Max liked to fly down the slide.

So when their mother, Laura Shatto, opened the door on that January afternoon, the toddlers — two newly adopted brothers from Russia — headed for the backyard with their three dogs bounding along. They were captivated by the swing set, with its bright blue slide, trampoline and glider.

Mrs. Shatto played with her sons for about 20 minutes, she recalled, before she had to run to the bathroom. She considered taking the boys inside, but it had been a stressful day for Max, with tears and tantrums. The backyard was fenced in. And it would just be a few minutes.

“Mama’s going to be right back,” she remembers reassuring them.

It was a split-second decision, she says, the kind of quick calculus that parents make all the time, weighing what seems like taking the smallest of risks against disrupting precious moments of peace. But when Mrs. Shatto returned, Max, 3, was lying in the grass, she said. He was not breathing.

In the next frantic minutes, Mrs. Shatto, then paramedics and emergency room physicians, tried unsuccessfully to revive the child. It seemed like a terrible accident — a severe allergy attack or perhaps a seizure — until the doctors saw the multicolored collage of bruises on Max’s body.