My Aspergers daughter never liked amusement parks and never rode the kiddie rides. We took her when she was about 9 years old. We had to shorten our day in the park to get her back home to the hotel so she could decompress. I dont think there is a way to desensitize. This is who they are.
Now she is 15 and we just got back from a Disney trip. We stayed in a condo complex with 3 bedrooms and internet so she could be comfortable while we went to the park. Really is wasnt any more expensive than a regular hotel room. She had a fridge with food and drink there and her laptop and movies. Plus it was gated and safe. Then I came home early from the amusement park while her sisters stayed late. Its a lot easier when they are older like mine so they can do thier own thing.
For your situation I would just say plan to break the day up with time for her to chill out at the hotel or your parents house. Or shorten the day for her. Dont force her to do rides and get help from family or friends who can either take her home or take her someplace quite and less crowded for a break. Oct is a good month for Disney with during the week being the best day to go as its less crowded. Epcot is a good choice since its a bit less noisy and with hustle and bustle. My daughter enjoyed Epcot but chose not to go to Universal. We went to Islands of adventure without her and was glad we did. The new Harry Potter park was crowded and it was hot that day. She would have been miserable.
My experinces is you have to adjust to accomodate them. I have no problem with leaving an amusement park early and going back to a hotel with a pool to just chill out.
The first time we went was for DS's 2nd birthday.
Disney is great!