Despite a soaring number of overweight children in the United States, many of their mothers don't know it. About a third of those with heavy children believe their kids are at a normal weight, according to a survey published in Pediatrics. Obesity researchers are alarmed that 20 to 30 percent of American children are either overweight or at risk for becoming so. Ask your child's health care provider for help if your child is overweight. Increasing your child's activity level and substituting healthful foods for junk and fast food can help.