We've got an childhood obesity epidemic in this country, so why not make bad versions of some of the offending foods to scare kids away from wanting to eat that type of thing, like for example, sugared cereals. Here are some possibilities:
Sugary Drudgery Cereal: Gives kids a jolt of energy so they can do chores or homework all day long! Ads would have an Oliver Twist-type character in a workhouse asking for more because the sugar rush has ended and now he's crashing.
Sweet Tooth Cereal: Crunchy frosted tooth-shaped cereal with marshmallow dental implements. Ads have a cartoon sadistic dentist character who says the cereal will give kids cavities so they'll have to get their teeth drilled for fillings!
Sweet & Sickening Cereal: The cereal that's sweet tasting, but treated with medicine to make people throw up, so kids would associate sugar and sweetness with nausea and vomiting. Think of how this could reverse the obesity epidemic! Ads would have a sick, green-faced cartoon character mascot who eats the cereal and projectile vomits on everyone. You can't say they didn't warn you!