Welcome and glad to hear about your DD's wish being granted.
Your DD will have a great time. WDW is quite accessible for someone in a wheelchair.
Click HERE for a link to the DIS site page about traveling in WDW with mobility disabilities. If you click all the way to the bottom, you will find a list of rides where she will be able to stay in her wheelchair for the whole ride.
My DD is now 16, can't stand or walk and is non-verbal, although she understands well and doesn't miss a thing. She can sit by herself, but doesn't have very good sitting balance. She has been going to WDW since she was a year old and has been on everything except Space Mountain, Big Thunder, Kali River Rapids and Rockin Roller Coaster. There are a few rides that are hard to lift someone on, but we have managed all of them until recently when she got to big and tall. For many rides, the seat is not really deep enough for you to hold a 9 yr old, but we have managed quite well with her sitting next to us, leaning against us, with an arm around her.
MK is the hardest park, IMHO, because most of the rides are not accessible with staying in the wheelchair. Since most of the rides are very short (2-3 minutes), you barely get settled and it is time to get out again. We like to take MK with a lot of rests and alternate things we have to take her out of wheelchair with things we don't. There is a Guidebook for Guests with Disabilities for each park that you can pick up at Guest Services. THey don't have alot of information about boarding rides, but they do list how long each ride is, which is helpful for planning.
Epcot is better, but it is an older park too, so not all the bathrooms are acceswsible for someone who can't get out of their wheelchair. The Studio and AK are both very good with accessibility, although people have complained about the fairly narrow and bumpy paths at AK.
If you have specific rides you know she is interested in, just post them and we will give you lots of info.