The facts are alarming: The number of overweight and obese children has more than doubled in the past two decades, nearly half of Americans from 12 to 21 are not regularly physically active and participation in all types of physical activity declines as age and grade in school increases, according to research from the University of Michigan.

But don't blame it all on the young folks. "Kids really like being active, but I think we've made it hard," says Dianne Ward, PhD, a professor and division director in the department of nutrition at the University of North Carolina at Chapel Hill. "We've built communities not to be activity friendly. Kids are secured in cars, in school programs and in homes that promote a sedentary lifestyle."

As kids head back to school and the carefree days of summer wind down — it's even more crucial to take a look at your child's fitness levels to determine whether she's getting enough action. How much is enough? The general consensus for grade-schoolers, tweens and teens is at least one hour of moderate-intensity physical activity each day.

"Parents have to take a leadership role in promoting family fitness," says Bob Harper, the life and fitness coach on NBC's Biggest Loser. "They need to embrace the concept that their kids need to be active."

If, like most families, you can barely make your jam-packed family schedule work as it is, we've chatted up experts for great ways to sneak in family fitness — and to make exercising fun — on a daily, weekly and monthly basis. Consider it this semester's homework assignment.