Kids can buy whatever they want if they have money. And surely, they'll buy the shiny thing thing they saw on TV or the internet when they encounter it in the shop and see it's cheap.
If you want the government to do your parenting for you, I don't know what to tell you. Not my idea of good parenting, and I think the results will be disappointing. We can agree to disagree on that.
Clearly you aren't a parent, and I suspect you also have never been a child. It would be handy if what you wrote were true. But children aren't robots, and parents aren't the only people who influence them. The reason that food companies target children in their advertising is that it works to manipulate them.