Obesity researchers are alarmed that 20% to 30% of American children are either overweight or at risk for becoming overweight. The percentage of children who are overweight has doubled in the past 20 years or so. And doctors are diagnosing more kids with type 2 diabetes, a disease linked to excess weight and seldom found in children until recently.

The CDC survey is the first large national study on parents' perceptions of their children's weight. The survey, in Pediatrics, included mothers of 5,500 children 2 to 11 years old. The children got physical exams, and mothers rated their kids' weight as too light, just about right or overweight.

In the study, two-thirds of mothers with overweight children said their kids weighed too much, but a third thought the children were at about the right weight. Mothers were most likely to correctly identify as overweight the heaviest and oldest kids, as well as daughters. Children at or above the 95th percentile for their age and height are considered overweight. From the 85th to 94th percentile they're at risk for overweight.

Mothers may believe that the youngest ones will outgrow baby fat, Maynard says. But childhood fat tends to linger into adulthood.

Mothers are most likely to notice the pounds on nearly overweight girls. Moms of daughters are almost three times more likely than mothers with sons to say their heavy kids are overweight. ''Sons may be held to a different, lower standard,'' Maynard says. So mothers may not feel as compelled to offer boys low-fat food or nutritional guidance. On the other hand, these gender differences could mean dieting pressure for girls, she says.

Dieting in childhood is destructive, says psychologist Ann Kearney-Cooke, an eating-disorders expert in Cincinnati. ''Six-year-olds should not be reading the fat content on labels. . . . This kind of thing leads to binges and eating disorders.'' Instead, she says, parents should emphasize physical activity and well-balanced diets.

Many kids indulge in junk foods because they haven't learned how to handle anxiety or depression ''without diving into the Doritos.'' Others spend too little time with their parents and are trying to nurture themselves with food, Kearney-Cooke adds.

The difference in how mothers see chubby boys and girls suggests ''there's still the feeling that a girl can't get the goodies in life if she doesn't look a certain way, and that's not true for boys,'' she says.