I brought my autistic son for the 1st time in 2005 - he was 9yrs old. He also suffers from sensosry issues and is high functioning.
We started preparing him by taking him to smaller amusement parks closer to home (Storyland, Santa's Village - Chuck E Cheese). He did have a hard time understanding that he was staying in a hotel room - but he had his lovey with him that helped (and his own pillow).
Doing a "story board/book" helps prepare for unfamiliar places also. The multitude of websites off pictures of just about every aspect of the trip to help prepare for what you will be doing and seeing. U-Tube can also offer video clips of rides & such.
My son's sensory therapist suggested bringing weighted bean bags to lay on his lap to help with sensory imput or mouth chewies. And we did need some quiet time. We stayed at Fort Wilderness camp ground in a cabin. Those rooms can get nice and dark/quiet when the curtans are shut - it was very helpful.
My son now is an expert on all things disney and is planning trips to
Disneyland Paris (yeah right!). He can negotiate Google Earth to view the parks all over the world.
By using the FastPass system we never had to use the Assist Card. Another great tip is when calling for your ADR's make sure they note any food preferences. Mine lives off of waffles & grilled cheese. Our last trip (August 2007) all the restuarants were well aware of his diet restrictions. The chefs each came out to meet him and often cooked more than (1) sandwhich for him. Disney really out does themselves for children with special requirements.
If characters are an issue - let the "handlers" know. My daughter also has disabilities and when some of the children were getting a little rough jostling to see a character, the handler came in and asked the other children to step back for a moment so she could see the cahracter without being knocked down.
If you have any more questions, please let me know or PM me - I would be happy to give any suggestions. I was very concerned my 1st trip because of bringing two children with special needs - I have NO HESITATIONS at all any more. Plan ahead, be familar and listien to your children.