We have taken our now 6 DS (autistic) and 3 (sensory disorder) the last two years in a row. I cannot begin to tell you of the advancements these children have made during our trips. This last one, they both picked up swimming, by the end of our trip they were swimming across the pool by themselves (with life jackets on).
As for the characters, I thought that they would be scared, they have had bad reacations to Chuck E Cheese and Santa, but on our first trip when they were 4 and 1, they couldn't get enough of them!. Word of advice, follow their lead, if they want to see them, let them go up, but also don't wait in a long line to see them. We loved the Character breakfast at the Contemporary, because the characters came to you and it was more of a relaxed setting. I got priceless pictures and they still remember them coming to see them. We found that taking big breaks in them middle of the day, really saved all of us. The were allowed to "de-stimulate" and were ready to go back for more after a nice nap or swim. I would even suggest that if your going to eat at the resort, go outside or eat at the pool area. The cafeterias at the All Stars and Pop can be loud and really taxing. We would go to Coronado Springs to eat a couple of times, it was less crowded and the boys really liked the atmosphere. I now it sounds really anal retentive but the more you plan and know, the better and easier trip you will have for everyone. I planned which day we would be in which park (what days had early entry) and what events we wanted to see in which park. Oh yes, and get at GAC pass, it was a lifesaver.
Hope this helps