I think what we're seeing, i.e. the rise of obesity in 'normal' children (as opposed to those with medical conditions) is related to changes in our society/culture.
As has been said in another post, when I grew up (60s/70s) the kind of food that we ate was very different. Fizzy drinks were a real treat, and used to be delivered by the Corona van (yes I'm that old!). Going out to eat or buying take away food was also something rare and special, by no means the norm as it is today for many of us (including myself).
We ate three meals a day and snacking in between meals was very rare; it just wasn't something that you thought about/expected to do.
Now we expect to be able to grab food whenever and wherever we are, just because we're a bit peckish, or because we fancy it. Just have a look at your local high stree next time you go out. How many of the shops are now food related? Coffee bars, take aways, restaurants, etc. HOw many of you will dash into a coffee shop and then pick up a muffin (or something similar) just because it's there?!
The other thing that is different is that we have lost our understanding of food and cooking. As a market researcher I am often researching foods and talking to people about cooking, and it is quite shocking how little people know about food.
So we no longer cook. Even using a cook in or pasta sauce these days is considered cooking by many people. (Not judging, btw, just stating a fact). Gravy granules are considered to be real gravy.
Therefore we are reliant on other people - restaurants, take-away restaurants, supermarkets and manufacturers - to make our food for us.
We all should take responsiblity for what we all eat (not just our children) but for many people these days that isn't so easy - they genuinely don't know where to start.