If you're doing counter service, $50 per day should be more than enough unless you have a huge appetite.
I end up spending a tiny bit less than I expect (provided we don't do a sit-down meal)--especially if it's hot, which impairs my appetite.
If you only eat 2 meals per day, counter service, with no snacks, drinks, or desserts, and are careful to get the cheaper things on each menu, $30 per day would probably cover you. (I find I often only eat 2 meals per day at the parks--the portions are pretty big. I think I've only eaten breakfast at the parks once in my last several trips; usually we're not hungry and don't want to waste the early park time!)
But if you add in a couple snacks or a third meal, you'd want to plan for $40+ per day to be safe. If you are not doing sit-down meals and if you are spending carefully, I think $50 per park day should be fine.
If I recall correctly, each snack, including popcorn and soft-serve ice cream, is in the ballpark of $4-$5. (Popcorn buckets, ice cream sundaes, candy apples, and other larger desserts would cost more.)
Have a great trip!