If i had a child who was diagnosed with crohn's or UC and really couldn't understand the disease very well I would absolutely make sure he or she was on a healthy diet, but I am an extreme believer in the power of diet in dealing with this disease. I realize its hard to make that decision for a child but it will help them so much down the road. If she ate whatever she wanted, she will most likely eat whatever she wants in the future and that will not bode well for her when she grows up. Being diagnosed at such a young age is hard because it is that many more years that she has to deal with growing up, if she continues to eat whatever she wants she could possibly have to keep having surgeries and that takes a toll on a body. Looking back, and picturing having this disease at such a young age, I'd rather grow up having my disease under control rather than eating whatever i want and having to take harsh medication and repeated surgeries.
Look in my signature for my opinion on the best diet for crohn's and UC patients. It is important however for a youngster to keep a balanced diet so not all diets you see on this forum will necessarily be in the best interest of your daughter, she still has a lot of growing to do.
I wish you and your daughter the best
Gianni