I had to wear a walking boot on a work trip a couple years ago. Just navigating the airport was quite a challenge and on the way home I had to get wheelchair assistance to help with that.
For Disney, if she was an adult I would recommend an
ECV, but since she is a child they won't rent them to her. So a wheelchair.
Renting from an offsite vendor will mean she has it at the resort and for the buses, which can be important becuase those can be some long walks.
I haven't rented from them yet, but will be renting from Gold Mobility Scooters ( ********************************/ ) for my next trip. There are threads about them in these forums and they have universally excellent reviews. They rent ECVs, strollers, wheelchairs.
Since she can walk, she should be fine with most rides.
When I was in my boot I got a waterproof cover for it from
Amazon. The one I got is this:
https://www.amazon.com/gp/product/B00LLLG9XY and it worked great. Note: It gets hot inside the boot without it and REALLY hot with it. BRING EXTRA of the "socks" you wear inside the boot. She will want to change them out as they are going to get all sweaty [and stinky]
And be prepared to wash the socks out each night -- in FL they may not dry overnight, so have at least one extra -- so bring some laundry detergent for that task. If you want to get fancy, a Scrubba wasg bag is great to take on trips -- no more using dubious hotel sinks for handwashing clothes:
https://thescrubba.com/ .
Finally, are you flying ? If so: my boot had air bladders in it that you fill to adjust the fit. Well, physics means those air bladders will inflate MORE as the plane ascends. Except I forgot that and got on the plane... As the plane was increase in altitude I suddenly felt lots of pain in the boot leg as the bladders expanded and put way too much pressure on me. I realized what was wrong and scrambled to deflate the air bladders, not an easy feat while seatbelted in an economy seat. Lesson learned: once you board the plane and are seated, deflate the air bladder [keep the boot itself on]. When you get to cruising altitude you can inflate it to the proper level. When the plane lands, adjust the inflation as it will have compressed and need more air before you go walking on it. Then you are good to go.
SW