We are a family of 2A and 2 kids aged 6 and 8. We've been to Disney 3 times in the last 3 years.
We bring $10 TOTAL (and we only use around $3) to buy the milk for our "in room" cold cereal breakfasts.
However, we DO bring a few snacks (granola bars, crackers, cookies--which we almost NEVER use) in the park bag along with bottled drinks brought from home. Oh, and we pack a box of cereal.
We've always had several snack credits left over that we use up on treats we bring home so even this (besides the box of cereal) isn't necessary but we find the magic lasts longer when we have mickey head crisped rice treats to munch on for the next week!
If we didn't bring these items from home, we'd budget around $50 total for the drinks and snacks and about $100 if we were eating breakfast in the food court.
My kids are average eaters but the excitement of Disney and the fact that we focus on keeping them well hydrated (lots of liquid in their tummies) leaves them with smaller appetites than they normally have.
We find that cereal in our rooms and perhaps one bottle of water or soda each is all we require on top of the DDP to keep us full and happy!
HTH!!