Yes, take your diagnosis letter, DLA award and a supporting letter (headed) from GP, dated within 3 months. Take it to City Hall, which is on the left as you go into the park.
McDonald's is in Disney Village if your DD will eat that - only just over the cost of a UK one. There are a lot of counter service places, so take your DD to the counter with you then find a seat. Eating lunch before 11am and after 2pm is recommended to avoid the big queues.
Take some ziplock bags to breakfast and 'procure' some bread rolls, cheese and ham, then that's lunch sorted
There are various water fountains throughout the park, to save on cost of drinks. We found a particularly quiet one beside the train station in Frontierland.
Pre-book for Mickey's Cafe, and if you are doing the buffet, ask when booking for a seat near the food area. The hotel is buffet too, I think so same would apply.
If you need to leave the table with her, and are afraid to leave your things at the table, please just ask! We found all the staff amazingly helpful and friendly - more so than the other guests 
There are lots of 'sandwich' and 'fast food' type places to eat, which are mostly cheaper than table service restaurants. You can find rough dining menus (and prices) here www.dlpguide.com/planning/dining/restaurant-menus/ - it's handy to make a list of acceptably priced places beforehand, so you have no surprises. On our list for this year, we have:
Table Service
Annette's Diner - Disney Village
Café des Cascadeurs - Backlot
King Ludwig's Castle - Disney Village
Planet Hollywood - Disney Village
The Lucky Nugget Saloon - Frontierland
and any of the buffet places.
Don't be afraid to flash your daughter's pass at any time you think it may help (especially for parades) as this often does mean she won't have to wait as long, or in certain conditions.
If you have any specific requirements RE: breakfast times etc, email the hotel beforehand, and tell the hotel when you check in. We asked for the earliest breakfast slot (7am) as DD has to eat before she takes her medication in the morning as it suppresses her appetite for the rest of the day - and it meant we could get our day started early to get to the park 
Most of all, have fun!!! When are you going?