I am very familiar with osteogenesis imperfecto type 2, the little one I know went as a MAW to DLR which was not the first choice but they thought it would be better for her.
She was able to do most of fantasyland and bugs life, I think that is all, and the shows and such. She was allowed to stay in the form seat that is shaped like her body and placed on the rides, but that maybe because of MAW, I am not sure, it is like her little brace it is just for her whole body. Of course she was 6 but only weight 13 pounds. She had a lot of fun.
I know of another little boy, I think he had type 4, his parents used a w/c to protect him in the crowds, he was older and of normal size and since his condition was no where near as debilitating as the first child I talked about, he was able to do more, such as POTC, and autotopia.
I believe it is really going to depend on what type he/she has and how much they can do before a break happens. After you figure that weigh in your options, DLR is compact, easy to maneuver, WDW is spread out and a lot more moving in crowds, and I don't mean in the parks but on buses, restaurants, and such, where in DLR you can stay on site, walk to and from parks and use restaurants off sites so less crowded. All choices will have pools to use and most have quiet pools that are a good option. I would say if you choose WDW I would go in sept or jan when crowds are low low. I would also do like many suggest with disabilities and use early morning hours to the max and then when crowds get big leave, come back late at night when crowds go down, even in the low seasons this will maximize your time, illuminate the need to rush, or be in crowds. I hope that helps .