For MK, if you are staying on site, you are much better off using the bus. The handicapped parking there is quite a hike from the TTC. Once you get to the TTC, you still need to ride a boat or monorail to get to the park turnstiles. The bus actually drops you off right near the turnstiles.
If you are on-site, you may find it's easier to just use Disney transportation. Depending on your vehicle, it may be kind of difficult for her to get in and out of the car/van. Almost 100% of the Disney buses are wheelchair accessible and she could stay right in the wheelchair.
Water Rides
Splash Mountain would be very tight to get her in and sitting, plus the side of the car is high (to keep the riders in). Getting in involves a long step over the side to stand on the seat, then a step down to get the feet where they belong. Getting out is even harder. Most people don't get that wet, but there is always the "lucky" person who gets soaked.
Kali River Rapids would be fairly easy to get in and out, but you either get wet or soaked - and there is no way to tell who will get soaked, it depends on which way the boat floats
Boarding
I think she'd be better off staying in the wheelchair for waiting in line. That way, she would not have to worry about having room to manouver, people bumping her and possibly getting her off balance. Besides that, some of the lines don't look that long, but have a lot of queue that doesn't show on the outside of the building.
I posted a link to a thread about boarding further up that should be helpful. Here's
a link to some pictures of ride cars from Deb Wills site that might help you decide which ride cars will be a tight fit. You can get a hard copy of the Guidebooks with Guests for Disabilities at Guest Services in any of the parks. It's not very helpful about exact information, but does give a general idea of whether something is a quiet boat ride or a turbulent thrill ride. The CMs at Guest Services might also have some helpful hints about rides where someone with a full leg cast will fit better.
Rough Rides
Some rides will bounce your child's leg around quite a bit. Star Tours, Body Wars and Dinosauer would have enough room for a leg cast, but might be too rough. Kilamonjari Safari is like riding on rough roads. Tower of Terror might be a problem - during the ride up, her leg might be "floating" and then drop down (jarring it) when the car descends.