I think there are way too many glib answers.
Childhood obesity is a complex problem, largely caused by the fact that we are more protective/engaged with our children than we were 30 years ago.. or 50 or whatever.. when kids roamed the streets from after school until dark, and parents didn't worry about what they got up to, so long as they weren't under foot, seen and not heard.
The exact opposite is now true, ironically we also over compensate for the lack of time we can spend with our kids too, by indulging them, we wrap them up in cotton wool, as we only have one or two, and we have them when we are older, and we over compensate for all the in-attention we got as kids, and we know the value of human life... or rather place a huge price on it, to the extent we would rather someone was alive, than alive and healthy.
Times change dramatically.
Our diets have changed dramatically.
Exercise patterns for kids have changed dramatically.
Blaming it on one thing or another is rather short-sighted, it is a complex intermingled issue, with complex solutions... what works for Jimmy, might be a total disaster for Jane etc...etc etc.
I hate glib answers to such a complex issue.
I was a very healthy weight child, incredibly active and sporty, my parents were OBSESSED with health, healthy eating, we only ever had (what even now would be considered) super healthy foods, and they were obsessed with my weight, to the extent they made me feel fat even when I clearly wasn't, weekly weigh in's and the recriminations if I was a pound or two over what they thought was acceptable. This was not an emotionally healthy place to be.
I am an overweight adult, despite that upbringing, and being well educated, despite knowing full well what makes you fat and what to do about it. It is a combination of a lot of factors, many of them well hidden and psychological in adults.
I have a kid who has his whole life been dramatically UNDER weight, as in until a few months ago, not even on the charts, and yet that is not taken as seriously as if he was the equivalent amount of overweight.. either will kill you just as quickly. Both are unhealthy, but I have had so many people tell me that it is fine he is so underweight, "at least he is not fat" >_< He will no doubt grow up with a horror of his family "forcing" or "nagging" him to eat his whole life. No doubt I have created in him some kind of demon he will have to feed with one bad habit or another when he is an adult.
Life is complex.