Oh Mark!
I am so super sorry to hear that! Well unfortunately that brings on a whole new ballgame for your trip. But not to worry - it can still be magical. The good news is that Disney is great with special needs guests.
I'm not sure what type of cast your daughter has but I would anticipate her needing a wheelchair in the theme parks during your visit. Disney has wheelchairs available for rent at each of the 4 theme parks & Downtown Disney. Click
here for a list of rental costs & locations. Advanced reservations are not accepted - they are rented on a first come basis.
Wheelchairs cannot leave the park from which they were rented. If you rent a wheelchair at one park & hop to another park the same day, you'll need to retain your receipt to show at the next park to get another wheelchair (if available).
If you'd like to guarantee that she'll have a wheelchair at the parks or need assistance outside the theme parks during your trip, you may want to consider renting one from a vendor in the Disney area. There are several vendors who cater to visitors to the Orlando area.
Your daughter will benefit from a Guest Assistance Card - click
here for more info on that. Depending on the type of cast she has, she may not be able to do some of the attractions where you get wet (i.e., Splash Mountain, Kali River Rapids). There may be other attractions that she may not be able to do with a cast so just prepare her for that so she's not disappointed if she can't.
I'd also check with her doctor for caring for her cast in the heat to make sure you keep her as comfortable as possible. Wishing you many magical memories :-) Allison