Hi Jennifer!
Thank you for bringing your question to the Disney Parks Moms Panel. One of the great things about Walt Disney World Resort is that they truly try and make a magical experience for every Guest and I'm glad to let you know that I think your granddaughter is still going to have a magical trip!
If she is in need of a wheelchair, you can either bring your own or there are
wheelchair rentals available at all Walt Disney World theme and water parks as well as Disney Springs, the map at the bottom of the rental page shows the various locations that you can pick them up. There is limited availability, but if you plan to arrive early this shouldn't be an issue! The pricing is subject to change and there is a deposit required, but I think you'll find the service to be rather convenient at keeping your little one of her cast.
A majority of the park attractions are equipped to make Guests with mobility concerns as comfortable as possible. These
Attraction Access Categories should give you a better idea of which rides she can stay in the wheel chair and which may require her to do a small amount of walking. As you can see the list is rather extensive, so she certainly won't be missing out on all the Disney fun!
Another option to consider is a
stroller rental in the parks as she may be more comfortable sitting in it throughout the day. I do want to let you know that there are no strollers in any of the queues, but some may allow you to stay in a wheelchair.
I'm sending pixie dust her way for a quick recovery and please let us know if you have any other questions before your trip.
-Lindsey