Hey Howdy Hey Arwen and welcome to planDisney!
So sorry to hear about your daughter recovering with a cast. This situation is a tricky one because a lot would depend on several things, the biggest being her height. One thing that I will strongly caution you on, if you do decide to take a trip down to Walt Disney World Resort while she is recovering, I would make sure you clear it with her doctor first as the attractions can be bumpy and may not be suitable for healing fractures or surgeries.
While attractions and ride vehicles are made to be as inclusive as possible, there are some requirements for knee mobility for some of the attractions. The more thrilling rides in Magic Kingdom like
Seven Dwarfs Mine Train and
Big Thunder Mountain Railroad do require quite a bit of knee mobility to ride those attractions. Slower-moving ones such as
Peter Pan's Flight and
Haunted Mansion should be better as there is much more legroom. There are also some attractions that have the ride vehicles in front of the attraction or if you talk with the greeting Cast Member they might allow you to try getting in and out of one to see if it will be appropriate for your daughter. So if you do decide on a trip to Walt Disney World, her ride selection might be very limited.
Sorry for the vague answer but there are many factors that will go into her ability to get in and out of the ride vehicles. I would again highly advise that you clear it with her doctor as they may be able to give you more guidance on what she may be able to tolerate and should avoid from a ride perspective. Good luck with your continued planning and let us know if you have any further questions.
Have a Magical Day,
Nick