Since the dining plan comes with snacks you could be okay depending on how much you snack.
Bottled water can take a big chunk out of your budget if you buy water at snack stands. The dining plan allows for a beverage with your dining meal, but bottled water would be considered a "snack" purchase if you purchase them at a cart and use the Dining Plan. Not worth using a snack credit for bottled water as water is only $3, as opposed to snacks that can be $5-$6 each. You can ask for ice water at any Quick Service restaurant. You don't have to wait on line. You can just go right up to the Pick Up counter and they have them there ready for guests.
As for souvenirs, I guess it depends on your kids. My older daughter is into clothing and jewelry, while my younger still likes plushes. The clothing is more expensive than toys, because she likes more than just a tee. I got her the more "fashionable" tops from Star Wars, Rock n Roller, Tower of Terror, etc, which run about $35 each.
We were there for 16 nights. According to my tally we spent $600 on souvenirs. This included random items like a souvenir $20 plush for my nephew, a $23 Chip mug for me, $18 on 2 Mickey icon hand towels, the Tower of Terror Clue game which was about $50, and $6 for a deck of cards to play one day while it was rainy by the pool. My younger daughter spent about $125. My older daughter spent $215. I have no idea what the balance of $140 was in purchases.
However, I actually purchased some of the items on
shopdisney.com. While we were in Disney they had 25% off code online, plus free shipping, so I ordered 2 of the tops, the hand towels, etc online and had them shipped to our house. I saved quite a bit.
Also, if you use the Disney Visa card for souvenir purchases at Disney World you save 10% if you spend $50.00. If my daughters saw something they wanted in one store and it didn't total $50 we would wait and purchase it in another store along with other items. That 10% adds up.