I'm sure I should respond to a lot more of your post, but I'm at work and only have a few minutes!
What I wanted to say, is tell Ivy to consider checking out some of the yoga studios and see what she thinks. Here in Canada, I would NEVER have thought of yoga as an "old people" things. Yoga studios are very popular and trendy (including hot yoga) and there are even many classes for kids. I've heard great things about the hot yoga, but haven't tried it myself.
Do you guys have Lulu Lemon in the US? It's a very popular store with the kids and I think they even offer classes in their stores. But, I think it originated in Vancouver, so it might not be in the US yet. Apparently their clothing makes your butt look great too!