Through my own experiences I feel that if the parents allow them to have cereal as a starter, then the choice between Frosted Flakes and Cinnamon Toast Crunch will be up to the kid.
Personally I came from a family that never had cereal for breakfast, so that advertising never affected me -- it's not like I'd ever have it.
I think its so children are open to eating it after the parents have bought it. If the food doesn't look appealing the parent will have to work harder but its the parents making the food choices not the child. Most of the time children don't even go to the grocery store with their parents.
So parents buy unhealthy food for their children without any prompting, and the kids need to have their minds opened to it by advertising, otherwise they wouldn't it?
Because most children aren't stupid and when something is called a "Fruit Loop" or the box displays pieces of cereal that resemble halloween candy they can make the connection to sugar.
If true why do cereal companies spend billions on advertising directly to children? For the fun of it?