Most parents I know, a lot of them moms, end up eating their kids' orders and food. Aside from licking what's left of the batter, picking on the morsels from the fudgies, and eating the rejects from the baked macaroons, etc.
That's how Moms end up resorting to the use of weight loss pills like Lipovox or special workouts. We're not careful about the extra we eat just because they're leftovers.
So when heaping portions on your child's plate... make sure it's just a single serving. You can always add more to the plate after. And when attending birthday parties, do not start on your food with your kid. Wait for his leftovers because chances are, it will be enough to fill you as well. You can then take home your share and eat it, not as an extra treat, but as a snack or main meal later. Same goes for ordering food.
If this scheme doesn't save your waist, it should at least save your wallet.