When you ask kids to exercise more and eat better/less in the name of obesity prevention, you're asking them to invest their time, energy, and self images into doing these things. And like any other investor, kids expect to see a return on their investment if their going to continue investing.

Increasing the Odds of Investment
In other words, if they can see regular documented proof that their investment is paying off, week after week after week, the odds of them continuing to invest is increased significantly. If the frequency of that improvement drops to month after month after month, some of the wind will drop out of their motivational sails. But if they have to wait three months, six months, or a year before seeing tangible proof that their investment is yielding a profit, odds are you'll lose them completely.
So the key to keeping a strong wind in their motivational sails is regular, frequent, documented progress - a tangible return on their investment of time, energy, and self image. However, if you can produce that kind of frequent and positive feedback, another factor comes into play. This factor I like to call The Compound Interest of Fitness.

Interest Compounding on Itself
Because of its unique ability to transform a little money into a lot of money, Albert Einstein, (among others) called compound interest "The eighth wonder of the world." It does this by allowing interest to be charged on interest, (interest compounding on itself) which grows the original asset exponentially. In this light, I contend that physical fitness has the same compounding effect on people... especially kids.

In the Physical World
In other words, show me a kindergartner who can run like the wind, jump like a rabbit, move quickly from side to side, climb a tree or a rope, and I'll show you a kindergartner who's brimming with self confidence. That confidence in turn spills over into their social relationships, their ability to try new things, including reading, writing, and arithmetic, and almost everything else in their young, impressionable lives. It's totally natural and in the genes.

On the other hand, show me a kindergartner who's unable to run, jump, move quickly, and climb trees and I'll show you a kindergartner who lacks the confidence of his naturally confident counterpart. And that lack of confidence also spills over into their social relationships, their ability to try new things, including reading, writing, and arithmetic, and almost everything else in their young, impressionable lives. That too is natural and in the genes.

And in the Lives of Our Kids
The moral of this story is that when young kids are exposed to experiences in which they get regular, and predictably positive returns on their investments of time, energy, and self image, they'll continue to invest (they'll relentlessly persist) week after week, month after month, and year after year. And if they relentlessly persist week after week, month after month, year after year they'll grow stronger and stronger at whatever it is they're investing in, whether it's their body, their social relationships, or their academic performance.

If we make sure that all kids are exposed to these kinds of regular, positive experiences we'll raise a generation of kids who will grow up to be strong, responsible, self reliant, and resilient human beings. As parents, educators, and citizens, how can we justify giving our kids anything less than a regular return on their investments, and helping those investments to earn boatloads of compound interest?