When we got to the sports doctor today for followup, he confirmed that it is in fact broken, and showed me one of the other angles of the xray that I think shows the break a little more easily.
When we got home, S wanted to get the dolphins on it right away. She wants to keep the cast forever, even after it's removed, but I told her she couldn't. Because of that, she got a little stingy with the dolphin stickers. She chose four to go on the cast, and is keeping the remaining eight. I think it needs at least two more; I may talk her in to adding more tomorrow.
I used mod podge to attach the stickers. I learned a couple things from the experience. First, I spread some mod podge around the area where the sticker was to go. I found it very helpful to get the stickers wet prior to applying- they laid a lot more flat that way. I think that fabric would go on a lot better than paper, but since dolphin fabric with dolphins of the right size is so scarce, we had to make do. Once we got the four dolphins on (and I may work on her to put more on there; we'll see), I put a light coat of mod podge everywhere, and shook some fine glitter on it. Unfortunately, that doesn't show up in this picture, but her reaction to her cast is obvious.
A close up of the cast.