I was a cereal addict as a kid. Killed a couple boxes of the stuff a week. And I was super skinny, probably because we played outside to the point of exhaustion every single day.
Yeah, was the same like like you as a kid. Was super skinny. Even as an adult, I'm not nearly as active but I do count my calories. IMO, banning things like cereal is not the answer when the more effective approach is to teach kids how to burn their calories and/or limit their calorie intake per day.
Kids' bodies work differently than adults'. Most of their caloric intake goes to the vastly more demanding process of growing than it does to fueling activity. It's like how a mother gains a huge appetite during pregnancy that would have made her fat otherwise.
The problem is that habits formed as kids die hard. You keep eating the same way as you did when you were 12, but have nowhere to put it all, except your belly / thighs / butt. Before you know it you're 27 and huge.