But in an interview, Dr. Katz also cited parents for “willful, genuine denial.”

Once a parent acknowledges the child has a problem, he said, “You have to deal with it.”

“ ‘Do I become the food police? Do I have to change my diet and walk the walk?’ ” he added. “So, often, it’s easier to pretend the problem’s not there.”

Other experts counter that the problem can be complicated and subtle, the result of family dynamics. Perhaps the parents are resigned to being overweight. Perhaps there are slender siblings, and the parents cannot figure out a diet that fits all.

“Denial can be a coping mechanism,” said Arnaldo Perez, a doctoral candidate at the University of Alberta who researches what motivates families to seek help for their overweight children. Before judging them outright, providers should explore parents’ possible feelings of guilt and failure, he said.

Denial may also be a form of wishful thinking.

It is “natural for a parent to want to think optimistically about their child,” Dr. Thomas N. Robinson, a professor of pediatrics and director of the Center for Healthy Weight at Stanford and Lucile Packard Children’s Hospital Stanford, wrote in an email.

“I have parents tell me that they waited to address it because they thought their child would ‘grow out’ of their extra weight,” he added.

It is only now, as Bonnie Ryan of Bridgeport, Conn., looks at old photographs of her grandson, 12, that she sees how the weight accumulated over the years. At 7, he was “chunky,” she remembers thinking. And at 8, chunkier still.