Kids aren't just fat slobs because of a high-carb diet -- they're fat slobs because they have a high-carb diet and don't do anything.
There's a good deal of truth to what you say. I know that kids still play, but I don't think they get nearly as much physical activity as we did in my younger years. I'd be curious to see a comparison of fitness levels between "gang-banger" type youth as compared to "video-game potato" type youth. I'd suspect that the bangers are more fit due to their being active. In comparison, the kids that spend a great deal of time playing video games and little time physically playing will tend to be soft, and often carry an inordinate amount of body fat even if they don't always appear overweight.
In fact, I suspect that there's a larger correlation between physical activity and obesity than between diet and obesity. Just a suspicion of mine, I offer no proof.