I also have a younger sister who has recently started wearing hijab full-time. I think that the easiest and most fun outfits for young girls are tunics over jeans, with a long sleeved top underneath if needed. Try to be positive when out shopping. For example, if she finds something you don't think is suitable, try to find a good alternative. Focus on what she can wear, rather than what she can't. In terms of hijabs, I think that an al-amira is the easiest for younger girls to wear, especially if she is quite active. Bright colours and fun prints are a must for young hijabis:
Pink al-amira £2.99, Simply Islam; print dress £8.80, Debenhams; canvas shoes £10, Barratts; flare jeans £11, BHS; blue al-amira £4.49, Muslim Base; aqua smocked dress £12-£16, Debenhams; canvas trainer £9, BHS.