I went to Disney 6 weeks after Achilles tendon reconstruction. 2 days before I had my cast removed and was put into my torture device...umm I mean my walking boot.

I was limited to what I could do but I was so happy I was in Disney it didn't matter.
I'm betting she will do just fine. Kids are so resilient. With having 3 weeks or so since she broke it I would imagine the swelling will be down for the most part so I think her pain level should be under control. Regardless any chance you get have her elevate the leg. Bring a hand held fan that someone can hold and let it blow on her toes to try and keep a bit cooler. Bring some trash bags with you to the park so if it starts to rain you can quickly tie one around her cast. If you find yourself without something to cover her cast go to one of the shops and ask for a medium or large bag. You can slip her leg in the bag and tie it above the cast.
One thing I did when I was in my boot is take pictures of my boot around the World. For instance when we were watching Illuminations I propped my boot up on the railing and took a picture of it with the show going off in the background. You could do something like this all over Disney World. (I just thought of a fun shot. Have someone hold a mickey bar by her cast. Or put mickey ears on her toes and take that pic) Then make an album with the pictures. I think she might find it a fun keepsake.
I hope y'all have a great time. Let us know how things went!