Thanks so much for the link to the trip report.
A few more details, now that I can give them. The family has decided to go to Disney World as a wish trip. That's awesome. I still want to be able to give them a picture of things they might have to be aware of as they get there.
A little more background - the girl is in what appears to be a large, reclining special needs stroller with ventilator attached to the back. It is probably 3-4 feet long, so I suspect that will be too long for most of the rides. I also highly doubt she will be able to leave the chair unless there is a ride where she can sit in the lap of one of her parents, as she has minimal ability to hold up her own head. Anything that the chair itself can roll on is probably a go so long as the ride car is big enough.
As long as her wheelchair with everything attached is no longer than 4 feet, it will not be too long for any of the wheelchair cars.
Some have things that stick up from the bottom, specifically the Aladdin ride and the dino cars on Triceratops Spin. Those are part of the attachment for the seat which is removable to make space for a wheelchair. If the wheelchair has anything that sticks out very low underneath that it may not clear the attachment point.
Another attraction that may be difficult is Toy Story Mania in the Studios. For that, my daughter's backpack usually needs to be removed from her wheelchair because the space is fairly small. There is also a spinning motion between different scenes which kind of whips you around so it may not be the best ride for someone with poor head control.
My daughter has been on all of the wheelchair accessible ride cars so if you need more information just Post your questions here. She can't stand but we can transfer her out so she has been on everything except the most wild roller coasters.
I also have pictures of all of the accessible ride cars at WDW. Some of them are posted on my DIS site photo album; others are still in my computer, but I hunt them down and post them when a specific picture is needed.
For the buses and some attractions, wheelchairs need to be fastened down. I'm assuming since she can't sit outside of her wheelchair that she probably has a chair with built in tiedown attachment points. This is not a problem on a standard wheelchair, because there are many points on the wheelchair frame that are strong enough for attaching the hooks. It is an issue on some of the more stroller like and reclining wheelchairs because unless they have specific tiedown loops manufactured in, they may not have safe spots where the large tiedown hooks can be attached.
She wants to do a princess meal, which I assume is totally doable. She's really into the princesses at this point, so is there any princess-themed area that will be inaccessible the family should know about in advance?
There are 2 Princess meals, at the Castle in Magic Kindom and in Norway in Epcot.
The one in the castle is upstairs, but accessible with a small elevator. That area has 2 seating levels - the one closest to the windows is down a few steps, so they would need to make sure to mention the wheelchair making reservations and checking in.
The one in Norway would be my preference - it is totally accessible and housed in a different sort of castle - quite mideval looking and was based on an ctual castle in Norway.
All of the Princess areas at any park are accessible. Other than rides which require a transfer that she would not be able to do, everything at WDW is quite accessible and they will find many things she will be able to enjoy.
Swiss Family Robinson Treehouse, Tomorrowland Transit Authority and Tom Sawyer Island are the only places that are not at all wheelchair accessible (stairs and moving ramp).
Also, she will need treatments once a day. The family is wondering if there is a place to bring this equipment and leave it (first aid?) so they can do these treatments in the park without having to trek back to GKTW.
There are lockers and there is a First Aid station in each park where they would be able to store equipment. First Aid also has treatment rooms where they could go to do her treatments.
