Welcome.
Just a few quick questions.....are you taking a car? Does your grandson wait well? How does he do on buses? Would a crowded bus make him feel overwhelmed? What is the budget? Is he the only child going or are there other children going as well? Is he a picky eater and/or have any food allergies? Does he do well in TS restaurants? These are very important issues when deciding on the best place to stay with an autistic child. I will tell you about my experience/situation and maybe some of it will apply to you.
My DD(5) is PDD-NOS and has a mild muscular myopathy so I can sympathize with the challenges you face. My DD also has no concept of time and has ALOT of trouble waiting in lines. She has many food allergies and is very picky about what she will allow in her mouth to eat b/c of sensory issues. She does not do well on crowded buses (found that out the hard way! Sorry to those of you who shared a few buses with us at WDW last May) Although she is sensory seeking, sometimes sensory overload can lead to crazy meltdowns and we have the need to "get away" swiftly to take her to a more familiar, safe, quiet place where she can unwind.
Last year, we chose the CR garden wing as it met many of our needs. The cost was not too bad(I got an AP and saved good money on the room and also got the DDE card and saved money on a few character meals. We are going again this year just within the one year time frame so I will save even more by using my AP and timing the vacations one year minus one week apart
), rooms are large and quiet w/soft, unbusy furnishings, nice uncrowded pool, lots of dining choices, monorail access to MK and Epcot, and with walking access to MK as well. It worked out great. When the kids were tired of the parks around 1 or 2ish everyday, it was a quick ride/walk back to the CR to relax in the room and pool. DH and I would order room service or grab food at the Food & Fun Center, and I would make sandwiches for the kids(got a grocery delivery from gardengrocer.com which worked out great and saved money on food that my kids won't eat anyway
). After dinner we would head back to the parks for fireworks or easy attraction riding since everyone else was waiting for the fireworks! Then it was a quick commute back to the CR to put the kids to bed. No waiting in line for long periods of time or walking a mile to our room.
This year we are trying the Poly as we have increased our budget and wanted to try a different resort. It also has great access to MK and Epcot thru monorail and boats. We have decided not to go to MGM as the kids were bored by it and we will just take a taxi to AK or anywhere that would otherwise be accessed by bus. My DD does not do well on buses and there is no need to put her or the other passengers in that kind of misery again
I have heard taxi rates are quite reasonable and much cheaper than a week of car rental that I don't need so wish me luck on that one
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I hope this helps. Let me know if you have any more questions. I am sure some others will post here and give you some of their experiences at other resorts as well.