My 7 year old dd is gluten free/casein free since she was 1-1/2. She has been to WDW 8 times. We have never had a problem getting gfcf for her. Of course she is a good eater and that helps. We always try to call the restaurant a day or two ahead when we can. But even with last minute stops the chefs are always willing to talk with you and give you guidance. We stay at DVC resorts and always bring some of her food with us on the plane in a soft sided cooler. We bring things like gfcf doughnuts and pretzels (a staple of all trips and useful on the plane ride). Last time we used wegoshop.com to order some groceries including Silk Soy. I always make the chefs understand that to a 7 yr old dessert is more important than the meal. If her siblings are at a make your own sundae station, the restaurant better have Tofutti or something like it to keep her happy.
In the parks she will sometimes get Itzadoozie (sp?) big swirly ice pop treat instead of a Mickey bar from the ice cream vendors. We have bought the pail of popcorn twice from the vendor in MK. I always ask about ingredients because you just never know if things change from one trip to the next. She always orders the Slushie (or whatever they call it) at Rain Forest Cafe. My dd is also a big fruit eater, so we always hit the fruit stands at Toon Town and MGM. My dd loves WDW and she loves to eat. She has done the Alice in Wonderland Tea at GF and the Pirate Cruise there as well-all gfcf. We also did the chocloate chip cookie baking at the Land (Dad ate the cookies she baked and we had her gfcf cookies on hand to substitute). Last Halloween I had heard there was a cookie decorating table at DTD, so I brought her gfcf cookies, frosting and decorations and set her up right next to her sister. Everyone was happy!
My dd is diagnosed PDD-NOS (autism spectrum disorder)and was very reclusive and panicky with an abundance of sensory needs. She just finished Kindergarten in a regular class (no aid-just speech once a week and ot once a week for handwriting). Her teacher said she would have never known that my dd had such a diagnosis if she hadn't read the reports. My dd has tons of friends and is very well liked by her peers. She participates in dance and gymnastics. Being gfcf was a big part of her success. Every now and again we mess up and we see the difference right away. Good luck to you!