I thought of food franchises because their advertising can be extremely persuasive to young children. Yes of course its reasonable for a company to try and sell their products, but the way they advertise for children relates to certain emotions, and when you relate any consumable with an emotion or escape from an emotion then addiction can develop (even with non addictive substance eg. alcohol).
Sure the parents have to give in to their kids but sometimes nagging can just becomes too much, you can't beat your kids anymore (I'm not supporting beating your kids) so you either have to try to endure it or give in. Not every one has an iron will.
I listed the government because of their lack of determination to do anything meaningful about this, they don't need to ban fast food but they could fund free school meals which are actually healthy, the whole healthy food gimmick is hardly healthy a lot of the time, in Australia school canteens only sell pies on certain days but they substitute with pasta, pasta is hardly a healthier choice your only replacing fat with energy and carbohydrates.
Natural behavior may also be influencing this, your body and genes don't know that we are no longer hunting for our food, so maybe thats why theres no kind of natural reaction to get up and run off the energy we just consumed, this kind of behavior needs to be programmed into our brains (good thing this programmed behavior easily overrules primal behavior, or otherwise we would never show any restraint).
People should be paying more attention to this issue, only really in the current generation has it become a problem, it is an epidemic in rich countries (terrible that rich countries die from heart attacks and develop diabetes from overeating while poor countries have people starving to death) and poses significant future problems to those societies. Search child obesity you might find out its a bit more serious than you thought, here is an article from a .org website it has UK statistics but still fairly relevant to other "western" countries (never cite .com websites, their goal is to get lots of hits so info might be made up to make more interesting) The Consequences of Childhood Overweight and Obesity