Sometimes...our kids don't look to the future.
For example, they might be, oh, gee, I don't know....stealing cookies at odd moments of the day.
Which is why you notice their belly is expanding faster than their height.
Kids don't quite understand the consequences for snarfing junk food...but the results soon become obvious.
Thus, tell your kids today that they're only sabotaging their own health if they abscond with candy and cookies etc....and then give them coping mechanisms for when the urge becomes too strong.
Halting this *now* before it become a huge problem...it's a good thing indeed.
ps - speaking about cookie jars, have you seen: