When it comes to helping an overweight or obese child slim down, parents routinely blow it, pediatric obesity experts say. Some resort to nagging or coercion, others put the child on a restricted diet, and still others issue sweeping bans against foods containing sugar or fat — tactics that are, at best, ineffective and, at worst, damaging.

Increased public attention to the problem of overweight — which affects one-third of Americans younger than 18 — has made more parents aware of the problem but has left them unsure of what to do. Should they intervene early, even in the preschool years, or keep quiet and practice a form of benign neglect, hoping that the baby fat will melt away as a child grows? At the same time, many parents are battling their own weight problems or hang-ups.

One Northern Virginia mother, who asked that her name be omitted because she wanted to protect her daughter’s privacy, said she found herself at a loss when her perenially chubby child grew visibly heavier than her middle school classmates. “I didn’t want to say the wrong thing and make her self-conscious, because she didn’t seem bothered by it. But I was definitely concerned this would be a problem, especially in high school,” the mother said.

“It’s not an easy place for parents to be at all,” said Eleanor Mackey, a clinical psychologist affiliated with the pediatric obesity clinic at Children’s National Medical Center in the District.

While some overweight children do slim down as they grow up, the likelihood decreases with age. An obese preschooler has an approximately 30 percent chance of becoming an obese adult, according to the Centers for Disease Control and Prevention, while an obese teenager has a 70 percent chance — 80 percent if one parent is obese.

Following are recommendations by pediatric obesity experts for parents concerned about what — and what not — to do.

The groundwork for obesity can be laid as early as infancy, said pediatrician Nazrat Mirza, who directs Children’s Hospital’s obesity clinic.

Many parents, she said, inadvertently overfeed infants, giving them six to eight ounces of formula or breast milk at a time, instead of the recommended three to four ounces. Others feed an infant every time the child cries — even though crying may be unrelated to hunger — or push food even after the baby has signaled he or she has had enough.

“People like fast-growing babies and regard a chubby baby as a healthy baby, but what they don’t realize is that they’re overriding innate metabolic cues,” Mirza said. Most babies and children younger than 4 or so instinctively know how much they need to eat. And insisting that children of any age finish everything on their plates can lead to habitual overeating, as can overly large portions.

Pediatrician Herschel Lessin, who practices in Poughkeepsie, N.Y., said that many parents misuse food as a reward or a bribe. “I have parents who buy their kids fast food five or six times a week,” said Lessin.

Adults, he said, commonly have skewed perceptions of children’s weight. “Parents worry way too much about a skinny kid and way too little about a fat kid,” Lessin observed.

Don’t focus on the scale

Too many parents zero in on the number on the scale — or proclaim that they want their child to lose a certain number of pounds — when they should be focusing on overall health.

“We really emphasize to the parents that the most important thing is to keep the focus on their child’s health, not their weight,” Mackey said. “The last thing we want is for parents to push weight dissatisfaction on kids and have kids feel bad about the way they look.” For a young child, these emotions, especially if combined with parental disapproval or disappointment, can be overwhelming.

Mackey said researchers have found that focusing specifically on weight loss rarely works for children and can trigger a vicious cycle in which distressed children turn to a reliable source of comfort: food.

Instead, she advises parents to say, “I really love you, and you have one body and one brain in this life, and I want you to take care of yourself,” and then discuss ways to be healthier, including an improved diet.

Hectoring a child can cause lasting harm. Mirza recalls the svelte mother who proclaimed, in front of her 5-year-old daughter, “She’s so obese.” The little girl burst into tears, lamenting that she was fat and ugly. “They had been hammering at her at home,” Mirza said.

Talking about a child’s weight in front of the child can “cause an over-focus on what the number means and on appearance,” said Washington nutritionist Elizabeth Davenport.

Davenport cautions that while parents need to be aware of a healthful weight, they shouldn’t overreact. Before a growth spurt or puberty, many children and preteens put on extra pounds, which come off as they mature. “It ends up evening out,” she said.

Forget special diets

Obesity experts see this often: An overweight child is put on a diet, while the rest of the family eats as it always did and high-calorie food remains in the house for thinner family members. Parents in such families sometimes protest that a skinny sibling should not be “penalized” by being denied chips or cookies or by having to drink nonfat milk, Mirza said.

But healthful eating is good for everyone, experts say, and should not be considered punishment.

“If it’s not adopted by the family, it’s not going to work” said Baltimore area pediatrician Daniel Levy, who founded the obesity task force of the Maryland chapter of the American Academy of Pediatrics. “You can’t ask a child to do something if you’re not willing to do it yourself.”

Imposing a special diet on one child, said Mackey, “is not sustainable. And children are very big into what’s fair.” Universal participation, she said, also “shows parents how hard it is.”

Parents who ban certain foods — sweets or french fries or pizza — are likely to find that the edict boomerangs. Such prohibitions can lead to hiding food or secret eating, which can mushroom into a full-blown eating disorder.

“These kids tend to go nuts when they’re around forbidden foods,” Mackey said. “One teenage girl told her mother, ‘I eat as much as I want when you’re not around because I never know when I’m going to get it again.’ ”

Learn basic nutrition

Endocrinologist David Ludwig, who directs the obesity clinic at Children’s Hospital Boston, said that many parents have major misconceptions about what constitutes a healthful diet.

“We regularly see parents who are well intentioned but have diets that have terrible nutritional quality,” said Ludwig, an associate professor at Harvard Medical School and author of a 2007 book entitled “Ending the Food Fight.” A surprising number of parents think that low-fat Twinkies or goldfish-shaped crackers and packaged macaroni and cheese are healthful, he said, even though they are laden with empty calories and minimal nutritional value. “There’s a lot of nutritional misunderstanding, much of it promoted by the food industry.”

Lessin, the Poughkeepsie pediatrician, cautions parents against giving their children juice (“sugar water”), soda (“a can a day is 15 pounds per year”) or fast food (“crack for toddlers”). If they want a child to lose weight, parents need to arm themselves with basic knowledge about proper nutrition, he said, adding, “There’s no magic.”

Emphasize activity

“A parent who says, ‘Go get some exercise’ while lying on the couch” is not likely to meet with success, Mirza said.

Kids are just as likely as adults to avoid exercise, but most value time with their parents, Davenport observed. So instead of telling a child to go out and play or get some exercise, do something together: Ride bikes, go for a walk or ice skating, or play ball.

“The first thing to do is to turn off the TV” and curb the widespread practice of putting a set in children’s bedrooms, Ludwig said. Studies have documented the common-sense observation that extended TV viewing and obesity are correlated. Kids who spend more than two hours per day watching TV or playing video games are more likely to be overweight than other children. And a 2009 report by the Kaiser Family Foundation found that nearly three-quarters of kids between 8 and 18 had a bedroom TV.

Don’t be the food police

“Do you really need another piece?”

It’s a question many parents have asked their overweight children; the desired answer is a foregone conclusion. Monitoring what an overweight child eats may be understandable, but it’s likely to ignite a battle for control.

Instead of saying, “That’s enough,” it’s preferable to ask a younger child, “Is there room in your stomach for more?” That teaches children to self-regulate and not rely on an external force that tells them to stop, Davenport noted.

In general it’s better for parents to refrain from commenting about how little — or how much — a child eats. Setting a good example, Mirza advised, is likely to be among the most effective anti-obesity strategies. “Small, sustainable change is our target,” she said.“Many times, we notice the child has become successful when the parents start to make changes.”