We just returned 2 days ago from our first trip since our DD8 was diagnosed T1D 18 months ago. I was so nervous, even though we have taken over 15 trips to WDW, but we did it! She did great- found she ran on the low side more often, but I am sure it was because of all the walking and pool time. I adjusted her long- lasting insulin a smidge, upped the carbs a bit at snack time, and that seemed to help. We tested about every two hours. Had no problems with her meter, but we packed a back up just in case. We used a FRIO- it was exactly what we needed to keep her Insulin pens, test strips, and Glucagon pen at room temp, so they wouldn't cook in the heat.
As far as carbs, all the snacks at the ice cream carts have nutrition labels, as does the grapes, milk and Uncrustable pb&j sandwiches you get in the the kids meals. Otherwise, we have become pretty good a estimating carbs in restaurant foods, based on chain restaurants who do post their nutritional values.
We did not get a GAC. With Fastpasses we had very few waits, and almost all lines anymore are shaded, or inside. If you have any specifif questions, please ask. The 2 weeks prior to leaving I went back and forth between "We got this, we can do it" and "What in the world are we doing, going to WDW in July with a diabetic for 10 days?"! But I knew we could not let Diabetes control us, and we needed to live our lives as normal as possible, just in a new way. We had a great time!