Just wanted to share more of my own experience, and what I've heard over the last couple of years as well. I too was so worried about Prednisone side effects. Particularly the common moon face that seems all too common. When first diagnosed, usually younger ones are pretty emaciated. As you had said, your daughter was losing weight steadily. I think you'll find that the effect on her face will be minimal. I actually thought my daughter looked better, even with the slight effect on her face. No one really took notice. It would be more predominant if you allow her to just eat and eat and gain way too much weight. Try to keep her eating to low residue foods and the healthiest choices possible. Watch the avoidance foods.
It will be interesting to see what the result of the colonoscopy brings. Most seem to have Crohn's localized to a particular area, but some can have it much more spread out. A co-worker's son, (15y.o), was diagnosed about a year before my daughter. His Crohn's was everywhere. After an initial round of Prednisone, he was prescribed Pentasa. He has had no recurrence, and has been taken off Pentasa, and is thriving.
As I indicated previously, don't worry about he size, as Crohn's can block nutrients from being absorbed. Once the inflammation is clear, everything starts getting through and she will really thrive. The one thing you might want to pay attention to is, her blood tests. I can only guess, that she is very likely anemic. Very common. This would explain why she doesn't have energy for sports and such. They will likely want her on an iron supplement, (Palafer). It takes quite some time to get the iron stores back up though, likely 6 months or more, but will make a huge difference in her energy levels.
Good luck, and keep asking questions, as I said, the people here are really good.