I Timothy 4:8
For bodily exercise profiteth little: but godliness is profitable unto all things, having promise of the life that now is, and of that which is to come.
In our world today, bodily exercise is a god. Having the perfect body and even just being "healthy" is the goal for many and we in the US spend, according to one source, 40 to 50 billion dollars on fitness and health products, clothing and gyms a year. You can't look through a magazine, watch the TV or browse the Internet without an ad for some product or service claiming to make you look or feel better.
The verse above is vital to our children, in that it shows us how to keep the fitness mania in a Biblical perspective. It doesn't tell us to just do what we want. Exercise is good; it profits little as the Bible says it, but the key here is, as with everything else in this life, it needs to be tempered with what is more profitable. Yes, it is good, but there is something better. It is a common problem to think that if there is something better, the first thing must be bad. Exercise is not bad, but there is something we should be spending more of our time on than that.
Godliness is profitable for both "the life that now its an of that which is to come." The time we spend on studying scripture, discipling our children, any way we can invest in the Kingdom of Christ, that is profitable for all things and all time.
So teach your children to exercise their bodies, but help them keep it in perspective and teach them with even more urgency, the importance of the exercise of Godliness. Show them by example how to invest their time and energy what will last. Built up their character and help them remember what will matter in and for eternity.