I agree with getting your own bottled water, it has saved my family a LOT of money over the years, best tip hands down!!
The other thing that has helped us over the years is to bring from home as many items as we logistically can, that would potentially cost us a lot if we had to buy them there, for instance, sunscreen and Advil/Tylenol. We always need those items and they're very expensive if purchased there. Sunglasses and hats are also pricey there.
Pre-buy souvenirs inexpensively at home, pack them, then pull them out and surprise your kiddos while you're there!
WalMart has Disney items at very reasonable prices, so does Target. Light-up bracelets/necklaces at the Dollar Store will save your wallet at night when the Disney-sold ones go for a minimum of $12 each! At Dollar Tree you can get a 3 pack for $1.
I have found children's meals to be plenty of food, many times when I'm at a counter service I will order a child's meal, I am small statured and a small meal is very satisfying to me. Just a thought.
Have a great time!!