I can't see how you read the article, connect that with the changes in the Hopi Indians, mention causes as changes in diet and exercise... And *still* come down on "Active kids will be less fat."
For one, the findings mentioned in the article claim that the kids have about the same overall activity during a day. The group that did sports didn't do much when they came home, whilst the group that did not do sports was a lot more active when they came home. Resulting in about the same overall amount of activity. If you missed this, you didn't read the FA.
Given this claim, and assuming the scientists know more about it than me, it seems reasonable that the remaining factor is the main factor. Which means DIET. Personally, I'm blaming the sugar industry. At least here in Europe, yogurt contains as much sugar as Coca Cola, about 10%. You will be hard pressed to find food without added sugar. And sugar wasn't really introduced like this into our food until 'recently', historically speaking.
The interesting thing is how studies find that in most cases, people prefer the taste of the food without added sugar. Problem is that those ingredients are more expensive than sugar.. Perhaps some of you with better bookmarks can dig those studies up again.
So, with this finding that *kids* have about the same activity level and *still* get obese, I claim bullshit on the whole "active kids will be less fat" theory.
Now, go spend some energy on opposing the sugar industry. And good luck with that.