If you're staying on site, I thinkthe DP is a good deal. We use a snack credit for a small item, then hit the CS's around 11 for lunch. During peak seasons, the CS's get packed (I mean, NO TABLE available, packed) so eating lunch early has an added benefit.
You are allowed to bring in (I believe) a small collapsable cooler; although we've havent done that since DD (now 20) was using a bottle. That would be great for small children as it could also hold juice boxes and individual packs of snacks.
When I stay offsite (inlaws own Vacay home), I leave that morning with a large bottle of water and a fruit/ power bar if we wanted to get to the parks early. Usually we will eat a quick breakfast in. During the day, I get tap water from CS restaurants.
-If you're driving to WDW, make a stop at one of the local grocery stores. We use the Publix store near 192, i think on South I-Drive and Osceola Pkwy. It's actually quite close to Pop. Load your room with items that you may be tempted to purchase from vending machines. If you're an adult beverage drinker, they sell beer and wine too.
- If you're flying use a grocery delivery service. There are several suggestions on the boards.
There are CS restaurants that are better values and offer more food than others. We like: Pecos Bills, Cosmic Rays in MK, Electric Umbrella in Epcot. Wolfgand Puck Express in Downtown Dis. For snacks, the Bakery in Paris actually offers a sandwich.
-If you do the DP it's possible to share meals
About spending money; yes, you will need to budget that as well; depending on your family's needs, resources, and expectations. As op's have said: tips, cab fare (sometimes taking a cab is so much easier!). If you're flying you may want to purchase airport food before boarding. Also, are you renting a car? Plan on filling up before returning the car (avoid gas stations near airport) and paying tolls on 417 and the Beeline Expressway. If we're particularly tired, we'll use a Bellman and tip $2/bag. We also tip mousekeeping, a few bucks a day.
-If you're driving, of course, gas, food, lodging.
Happy Planning!