I don't think I've ever heard of a CM telling someone they couldn't ride because of a cast; ultimately it's your decision. As she's so young, she's not going to be going on any of the thrill rides, which are the only ones I can see being a problem with a cast. I don't think that going on It's a Small World is going to do her much harm.

Besides, most children her age will be spending a lot of their trip in a stroller too, so it probably won't be that much different for you than if she didn't have a cast.
I don't know if anyone's told you about this yet, but you might want to look into getting a Guest Assistance Card (GAC) to allow you to use a stroller as a wheelchair. This will mean that you can take her stroller into lines and buildings such as The Land, where they're not normally permitted. For more information on the GAC, see the
FAQ thread, post 6.
When using this, you will be routed through the wheelchair entrances. Most of the time, this is the same entrance that everyone else uses, though you may be pulled out of the queue near the boarding area, to board elsewhere (usually the exit). For some rides at MK and EPCOT, you will go through a different entrance. You will normally wait about as long as everyone else, but at least you won't have to be holding her for all that time. Whenever you get to a ride, just ask the CM at the entrance where you should go with a wheelchair, and they'll direct you!
I say go and have a great trip!