Hi Helen,
Welcome to the Disney Parks Moms Panel! I'm so sorry your 10-year-old has an ankle injury that will require a cast or a walking boot for your Walt Disney World Resort vacation. However, I do feel uniquely qualified to answer this question because I visited Walt Disney World Resort with an ankle injury requiring a walking boot just last year.
At ten years old your options for getting around the parks with a mobility issue are limited. There is either walking or getting pushed in a wheelchair. It can be surprising how much of a difference the cast or walking boot can make on your energy level. Fortunately, my friends and family were willing to slow down and take a break when I needed it. I highly recommend visiting one of the
First Aid centers for a little break. They are a wonderful, peaceful place to sit for a bit and regain some energy.
I don't think the cast or boot will get in the way of any of the attractions except one. Avatar Flight of Passage in Disney's Animal Kingdom has a unique ride vehicle and one of the restraints comes up behind your calves. I didn't have a problem with the walking boot (and I don't have a small calf) but I would check with the Cast Member before you enter the line to make sure a cast won't be a problem. They have a test vehicle out front so you can discover any issues before you get in line. There were some attractions, like Space Mountain, I found a bit awkward to get in and out of with the walking boot on. You might also want to bring a plastic bag to cover the cast for any rides where there might be a chance of a drenching like Kali River Rapids or Splash Mountain. Hopefully, your ten-year-old will be able to happily visit many attractions.
Wishing you a magical vacation,
Amy H.