It really depends on what you plan to eat, also.
Some suggestions I have:
SInce you are staying off property, I am assuming your are driving in. If so, grab a big breakfast somewhere off property. You have McDonald's, Perkins, Bob Evans, etc. all within a quick jaunt. This can get you through the morning easily without spending a ton. Then, grab a later lunch. You will beat the rush and probably cut back on snack necessity.
As for meals, and average adult lunch after everything is said and done about $10 a person. Snacks run about $2.00-$3.00 each item (sodas, popcorn, etc.). You can get out fairly inexpensively on food alone for lunch and snacks. Also, SHARE! It's really a good way to go for these. We would go (3 of us) and grab a snack in the morning, one thing and two drinks. Just split them. Then do the same thing later in the afternoon. It gets your hunger fix without everyone having to get a big thing of food.
Souvenirs are really up to you. Expect to spend at least $20 on each child, most likely. Probably will be more.
So, grand totalling that for lunch, an afternoon snack, and souvenirs, I would say you can scrimp with $100, but $125 to $140 will probably be more reasonable for EVERYTHING.