Don't forget, exercise comes in many forms.

"When most adults think about exercise, they imagine working out in the gym on a treadmill or lifting weights," according to KidsHealth.org. "But for children, exercise means playing and being physically active. Kids exercise when they have gym class at school, soccer practice or dance class. They're also exercising when they're at recess, riding bikes or playing tag."

Cohen said 30 minutes of cardiovascular activity three times a week is a good target for which to aim. And families should keep daily screen time to less than an hour a day on school days, whether that means videogames, TV or non-school-related computer surfing.

While a family can initiate its own fitness program, Cohen recommends seeing a doctor before you start. In his practice, Cohen asks questions to get a complete picture of the family's overall health, and runs screening tests on the children that may pick up hidden problems.

A doctor can help a family tailor its fitness regimen to address any problems. The benefits are substantial and cumulative, and can help increase life expectancy and lower the chance of heart disease.

"Increased physical activity has been associated with an increased life expectancy and decreased risk of cardiovascular disease," according to the American Heart Association. "Physical activity produces overall physical, psychological and social benefits. Inactive children are likely to become inactive adults. And physical activity helps with controlling weight, reducing blood pressure, raising HDL ("good") cholesterol and reducing the risk of diabetes and some kinds of cancer."