I have gone on an annual trip to WDW with dd who has ASD, since she was three, and a few strategies have really helped to make it less overwhelming for us- you probably already know most of it, but here goes:
-down time in the room with favorite toys/books/tv shows after a plane trip, to allow her to "decompress" from stress of traveling, and every afternoon for a break from the parks. Pool time is great, too! Just a quiet evening on the first day is ideal, with an early bedtime.
-we did frequent healthy small meals and snacks, with plenty of liquids at all times, with sweets limited to just one or two small "treats" a day really increased her ability to cope with the overstimulation of parks. DD wears her own fanny pack with nuts, pb crackers, cheese sticks, and water, which she can "graze" from as needed. I can't stress enough how much this has helped us!
-we approach the parks with a relaxed attitude, stopping to enjoy whatever catches her interest, doing low-key rides that she chooses, no pressure to do "big" rides, and time to just "stop and smell the roses"- or feed the ducks!
-I try to have my daughter wear a swimsuit under her park clothes, and sandals, so she can enjoy the interactive fountains, or pack an extra pair of clothes/socks/shoes, and SUNSCREEN! We use Bullfrogs spray sunblock- good for most of the day, plus so easy to put on!
-we avoid high stim rides: at MK, for dd, these include Stitch's Great Escape, Mad Tea Party, Snow White's Scary Adventures, all 3-D type attractions as well (until she was older, and now she does pretty well with Mickey's Philharmagic),Pirates', HM, Splash, BTMRR, etc.
-bring or
rent a stroller!
-when she was younger, we took frequent breaks between rides/stroller time for snacks, or letting dd blow off some steam at a play area such as Donalds Boat, Tom Sawyers Island, the slide area next to Splash Mountain- now there's Poohs Playful Spot, too!
-we try to keep as close to her usual sleep schedule as possible, even if it means we miss out on some evening events. And at the first sign of overwhelm/meltdown, we head out of the parks for a break in room, pool, beach,boat ride, monorail ride, etc.
-watch fireworks from the Poly beach first, to see how she handles the noise.
-wearing headphones that cover her whole ear helps dd with loud rides- which seem to be most of them (at least to my dd), loud parades, fireworks, noisy restaurants/airplane rides/airports, etc. This took some practice at home first, to get used to them- or earplugs, if your dgd will wear them. Maybe pratice at home, then the mall or grocery store, etc. She may not like them, but its worth a try. They have really helped my dd- we just use an older headset, with the cord cut off. But don't stress over this if she won't wear them- maybe when she's older.
-my dd still prefers watching the characters from a "safe" distance
-favorite low key rides at MK for dd since she was little include: Walt Disney Railroad, Swiss Family Treehouse, Alladins Magic Carpets, Tom Sawyers Island, It's a Small World (not any more!), Dumbo the Flying Elephant, Cinderella's Golden Carousel, Minnie and Mickey's House, Donald's Boat, Carousel of Progress, and Tomorrowland Transit Authority.
-we go to EMH in the mornings, or right at opening, avoid lines more than 10-15 min, keep small toys on hand to keep busy in line (bubbles were a favorite), and try to leave before she is loosing it- usually only about 3 hours in the am, maybe 3 hours in pm- maybe not!
-she was also overwhelmed by "moderate" attractions including Illuminations, Fantasmic!, Muppets 3-D (luckily we talked ahead about where the exit was, told her she could ask to leave any time- and we did, after first two minutes!), Festival of the Lion King, and Maelstrom.
Hope you have a magical trip!
Roo