My goodness, I am so sorry about your daughter's foot. I hope she's not in too much pain.
I was able to find several Orlando area services that rent pediatric-sized wheelchairs:
- medicaltravel.org
- discountmobilityusa.com
- orlandovacationmobility.com
You may also find the post
HERE helpful as it discusses larger mobility issues at WDW. You should absolutely read the allears.net and get the Open Mouse book mentioned in that post. These will go a long way to helping you navigate WDW with a cast.
Also, my daughter had a cast on her arm during one of our WDW trips. We had our orthopedist fit her with a waterproof cast liner so that she could go swimming at our hotel pool. I don't know if this is medically appropriate for you daughter's situation, but you may want to ask your doctor if he or she can do this. It might make your daughter feel a bit less left out of the fun.
Good luck!