hey have several different size chairs, I saw child size,
adult (narrow) and adult (wide).
Child size wheelchairs from WDW must be rare or new. I've read in the past that they didn't have them and I've never seen anyone in one from the resorts or parks, just from off-site renatls. People have also posted that what they needed and what was available at their resort were 2 differnt things, so you would be safer off renting from outside (or at least take the contact information with you). She may need to elevate her foot; you could get a wheelchair from off-site that would handle that.
Parking: If you don't get a temporary handicapped permit from your state before you go, tell each parking lot CM you see that you have a wheelchair. They usually try to save some spots close to the front on the ends of rows for people in that situation.
For ride access: In most cases, people with wheelchairs just wait in the regular line (that's called Mainstream Access) until they get to a point where wheelchair can't go (like stairs or a narrow turnstile without a gate). At that point, there will be a gate with the wheelchair symbol on it and a CM will direct you just because you are using a wheelchair (no special permision needed). They will try to keep your party together, but the official information is the guest in the wheelchair plus 5 members of their party can use the wheelchair entrance.
Most people assume when they see someone in a wheelchair, that they are being boarded when they disappear from the line. Usually, you are going to the exit to finish your wait because it is the only place where someone with mobility problems can board. That's another reason they limmit the size of the wheelchair party; some of the exits have small spaces for waiting. Because of fire rules, they also have limits on how many people with special needs for evacuation can be on a ride at the same time. You often will have an extra wait, especially if you must stay in the wheelchair; either to wait for the wheelchair car, to wait for a CM to assist you. For example, at Buzz Lightyear, we usually end up waiting at the exit for several ride cycles and often see the people who were just ahead of us in line (before we went to the exit) finish their ride and get off before we have gotten on. There are some rides (like Haunted Mansion) where you will miss a large part of the attraction - you miss the entire "shrinking room" and only go on the ride car part.
All the attractions at the Studio and AK are Mainstream Access. Because MK was built a long time ago, it has more rides that are not Mainstream, but many are. Epcot is mostly Mainstream.
Your niece will still be able to go on most rides even with a cast. You can get a Guidebook for Guests with Disabilities book for each park form Guest Services or
click here and you should find a link to guidebooks you can download from the official Disney site. You don't need to remember all the info in the books, the CMs will direct you.
Anyway, have fun.