Fast-food restaurants are in on the act, and over the last two years have begun to introduce their own mini-meals, like the McDonald’s Snack Wrap. According to the Agriculture Department, American children get 40 percent of their calories from food of poor nutritional quality.

What is especially baffling where I live, in Los Angeles, is how often the kind of parental paranoia that obsesses about school ratings, vaccines and myriad imagined plagues is matched by utter disregard for the nutritional downsides of mowing down Fruit by the Foot every afternoon at 4. Rarely do I see a parent show up on the soccer field with a homemade snack, or even a bag of carrots. Oreos are the post-game snack of choice, even in sports leagues dominated by upper-income parents.

“There is definitely a big disconnect,” said Dr. Howard Taras, a pediatrics professor at the University of California, San Diego, who specializes in community and school health policies. “I think there is this natural tendency among parents to not want their child to go hungry. It is more difficult for them to think about the long-term outlook for the child.”

Ellyn Satter, a dietitian and family therapist, thinks there is little point in worrying too much about what children are eating  unless it is “pure sugar,” like juice, she said  or how much, because children self-regulate portions. The key, she said, is to end grazing. “The parents’ job is to do the what, when and where of feeding,” she said, “and it is up to the children to do the how much and whether of eating. In order to have successful family meals, you have to structure the snacks.”

Carolynne Dyner sees the purpose of snacks for her children, Quinn, who is 7, and Sadie, 5, through a fairly simple prism. “To stave off tantrums, of course,” she said. From their days caring for infants, she said, parents are conditioned to be prepared for a sudden attack of hunger. And so she keeps her car and purse amply packed with pretzels, baggies of Cinnamon Life cereal, Goldfish crackers and Clif bars.

For her children, little bites between meals have in some ways supplanted the meals themselves. “They usually need a snack midmorning and midafternoon,” explained Ms. Dyner, who lives in Beverly Hills, Calif. “There may be a third snack, and this is usually due to the fact that our kids didn’t care much for what we provided for dinner, so now it is 7:30 and they are hungry. At this point we may give them a yogurt.”

Parents who give in too many times may find that snacks are the culinary equivalent of letting your 2-year-old sleep in your bed. “People get themselves into these habits, which they later regret tremendously,” said Ms. Ikeda, the nutritionist. “We do, as parents, make mistakes and then we either have to live with them or suffer the consequences in fixing them. It gets exhausting saying ‘no’ all the time.”