My suggestion would be to find out what each person who is going thinks would be special, rather than trying to come up with some special activity. And, then try to include at least one of the things that each person wants to do if possible.
Some people have posted about how they had some amazing plans, but their child ended up not really caring about that, but LOVED the monorail or the train and would have ridden either of those for hours and hours.
I agree with the others that just being in the park may be quite overwhelming, especially if none of them have been to WDW before. The Photopass idea is good, but buying a Photopass is kind of expensive and none of them may have the money to do that (even sharing the cost). The Photopass photographers will take a picture with your own camera for you, so that you can get pictures of the whole group. The scrapbooking idea is great and will help them to talk about the experience and share it with others when they get home.
The disABILITIES FAQs thread can help you out with some information. That thread is toward the top of this board, or you can use the link in my signature to get there.
There is information about Guest Assistance Cards in post #6 of that thread and you will also find information about specific conditions/concerns in post 3 with access information in posts 11, 18, 19, 20 and 21.
It would be helpful to have more information about the types of disabilities the people have in order to give more suggestions. Things like whether they also have physical disabilities, can they all walk enough to transfer to attractions, what level of understanding they have, etc??