I see. So someone who is underage falls into different categories just because he or she is young. That, and you personally surveyed the fat population in its entirety.Under most states law children are sometimes seen more as property of their parents. Plus they can't vote. So their Representatives or Senators are a lot less likely to consider their opinions. And again those kids want that fat food, so technically everything is all right legally.
I know of cases where children were fed fast food only because it was a cheap option, not because they wanted to eat it on a daily basis. Moreover, you conviniently mistake the argument and transcribe it from "regulating diets of children and imposing rules on the parents until the child can decide what diet it wants to follow" to "letting kids eat what they want".
They are the property of their parents with little rights, but they have the right to choose what to eat? Contrasts and double standards, I say.