It's off! After four long weeks, our little girl's cast's has been removed, and of course, retained for posterity. I'm expecting it'll be added to the bin we have of mementos that we're saving for her and her brother. The follow up x-ray indicates all has healed well, and beyond a moderate amount of discomfort over the next few days as she resumes use of it, we can call this chapter closed.
Oh, and on the way to getting it removed, her brother sincerely declared that he'll be sure to not knock her over and fracture her arm again.