If you want to save money, I recommend NOT doing the dining plan, unless you and your family really, really want to have daily table-service meals, character meals or dinner shows as part of your experience. (If you're wondering about that, I suggest doing the math and comparing the cost of the dining plan with the cost of what you would normally order: dining plan costs and WDW restaurant menus are accessible online).
For a short stay like yours, if you want table service I'd suggest booking only one or two table-service meals. Lunch is typically cheaper than dinner, and you can find the best venues just by googling, "least expensive table service at disney world" or "best value table service," etc. There are similar lists available for counter-service meals, along with online tips about counter-service meals big enough for two to share (e.g., chicken and ribs at Cosmic Ray's).
I recommend that you bring all of your snacks and drinks yourself, and as another poster noted, you can get ice water for free, on request, at any counter-service venue at any time. If you want to save even more, bring your own cereal (you can buy milk on-site and keep it in your room fridge) or Pop Tarts, and eat breakfast in the room. If meals in the parks aren't in the budget at all, you can pack your own lunches (PB&J with some fruit and chips, etc.) and bring them with you to the park.
The first couple of times my family went to WDW (when I was a kid), my parents were on a shoestring budget. We camped in the campground, had cereal for breakfast and sandwiches and soup for lunch and dinner, and didn't spend a penny on food in the parks, except for some ice cream treats during the fireworks on our last night. Although I really appreciate that I now have the luxury of buying lunch and dinner in the park, I certainly never felt like I was "missing out" as a kid, and the less time and money you spend on food, the more time you have to enjoy all the rest that the parks have to offer.
