Everyone loves a chubby baby. We crow about what a good eater he is and blow raspberry kisses on that Buddha belly. And why not? Babies should have pinchable cheeks and dimpled knees—their main activities are snoozing and slurping down high-fat milk. But when that chubby baby becomes a roly-poly toddler, and then a stocky preschooler, it can make you wonder: When are those double chins just baby fat, and when do they become, well, fat?

That’s a good question to ask, and not just for vanity’s sake. Childhood obesity is reaching epidemic proportions in the United States–the latest studies show that nearly one in five preschoolers is tipping the scales at a dangerously high weight–and the health repercussions of carrying around all those extra pounds are anything but cute. Obese kids have a higher risk of developing type 2 diabetes; the Centers for Disease Control and Prevention predicts that approximately one in three kids born will develop the disease. Obese kids are also at increased risk for liver disease, high blood pressure, and cardiovascular disease–illnesses we usually associate with middle-age smokers, not apple-cheeked grade-schoolers– not to mention joint problems from too much pressure on the hips and knees.

No mom or dad would ever wish any of those health problems on their kids, but American parents are probably the world’s worst judges of their children’s weight. A national poll from the University of Michigan’s C.S. Mott Children’s Hospital found that only 13 percent of parents of obese kids recognize that their children have a problem, and the heavier the parents are, the more likely they are to think their chubby offspring are just right. Of course, all parents view their kids through love’s rose-colored glasses, but there’s more to it than that: “Obesity is getting normalized in our culture,” says Sandra Hassink, M.D., director of the weight management clinic at the DuPont Hospital for Children in Wilmington, Delaware. “Parents have trouble determining if their child is overweight or obese just by looking.”

That’s why the best way to assess your child’s weight is not by how round his face is or what size T-shirt he wears but by watching the numbers on his growth chart, says Roberta Anding, R.D., a pediatric registered dietitian at Texas Children’s Hospital and one of the editors of The Family Guide to Fighting Fat. Focus on the ratio of weight to height, also known as the body mass index (BMI). “If the child’s weight is increasing at a faster rate than his height, that’s a red flag,” Anding says. A BMI above the 85th percentile is considered overweight; above the 95th, obese.

Also think about the kinds of eating and playing habits you want to hand down to your kids. Remember, they’ll inevitably mimic whatever you do–whether it’s planting yourself in front of a Real Housewives marathon or planting a garden in the yard. To get you started, here are some simple but potent changes you can make to keep your entire family at a healthy size:

Watch what they drink

In the past, kids came home from school and had a nice cold glass of milk; today, they’re just as likely to toss back a sugary juice box or soda loaded with empty calories. Instead, offer her water mixed with a dash of 100 percent fruit juice for flavor. And if your child is older than 2, graduate from whole milk to 1 percent to skim.

Downsize your dishes

Over the past couple of decades, portion sizes have exploded–not just in supersize restaurant meals but in our homes as well. Anding suggests serving your child on a salad plate; fill one half with an entree, such as pasta or chicken, and the other half with fruits and vegetables.

Introduce Whole Foods

“Kids who grow up on highly processed and fast food learn to expect things to taste very salty, sweet, and fatty,” says Suzanne Rostler, R.D., a nutritionist at Children’s Hospital Boston. “If you train your child at an early age to love the taste of whole foods, that’s what they’ll be more likely to want.”

Get moving

All that healthy food will take you only so far if your kid sits on the sofa like a slug–and according to the CDC, 23 percent of American kids between the ages of 9 and 13 get no exercise at all. So give your child plenty of chances to burn off energy every day by playing Pirates vs. Jedis in the yard or setting up an indoor obstacle course on rainy days. And try walking, scootering, or biking places together instead of piling into the combi.

Play detective at daycare

Even if you serve your child all the healthiest foods at home, your daycare providers or babysitters may have other ideas. “One mom told me she stopped by daycare and saw her 9-month-old with a french fry dangling out of his mouth,” Hassink recalls. Ask your care provider to spell out what your child is chowing down each day, and send along snacks if you need to. And make sure that a good portion of the day is spent in energy-burning play.

Limit TV time

Clicking the remote from SpongeBob to Hannah Montana does not count as exercise. One study of 5-year-olds found that for every hour watched above the American Academy of Pediatrics’ recommended limit of two hours a day, a child had a 7 percent greater risk of being obese by age 30. “It’s not just the sedentary time–it’s all those commercials for food,” Hassink points out. A study in Pediatrics found that nearly 98 percent of food ads viewed by kids between ages 2 and 11 were for junk high in fat, sugar, and/or sodium. Says Hassink, “The counter-programming by the ads is going to be much more powerful than anything you can do, unless you’re a cartoon character.”

Eat dinner together

It’s great to have someone else do the cooking and wash the dishes, but if you use the local pasta joint or fast-food chain as your family caterer, your kid’s health will suffer: One study found that when children eat out, they consume nearly twice as many calories as when they eat at home, plus more fat, sugar, and carbs. A meal at home with the family around the table has the opposite effect: Kids eat one and a half times as many fruits and vegetables as when they eat by themselves, and they tend to continue to make such healthy choices throughout the day, according to a Harvard study.

Practice what you preach

There’s no better motivation to get healthy than being a parent, since your kids are always watching. So be a good example: Exercise together (play tag, or take up a sport in tandem), pick out favorite veggies at the market as a family, and don’t keep a lot of unhealthy foods around the house. Hey, there’s a payoff: While you teach your kids habits that will pay dividends throughout their lives, you might also wind up finally getting rid of your own baby fat!