Sorry to hear about her brace, but you will still have a good time.
If you follow the link in my signature to the disABILITIES FAQs thread, you will find a lot of helpful information. (That thread is stuck near the top of this board, if you want to look at it again).
Post #2 of the disABILITIES FAQs thread has information about renting wheelchairs, including links and phone numbers for off-site places that rent wheelchairs.
If your daughter is a small 12 yr old, she will probably be more comfortable in a narrower wheelchair than most of the companies rent.
Apple rents 16 inch wide wheelchairs, which are quite a bit narrower than the 18-20 inch wide ones that most of the other companies rent.
(For a bit of perspective, my youngest DD is 5 feet tall, weighs about 83 pounds and is very slim. Her personal wheelchair has a 14 inch wide seat).
The disABILITIES FAQs thread also has some other information that will be helpful for you, including a list of attractions that have health warnings.
Post 1 of that thread is an index to which post has which information.
Also, keep in mind that if your daughter needs to lie down for a while or needs to adjust or remove her brace, you can use First Aid in any park. They have private rooms or cubicles with cots and are open in each park as long as the park is.