This is a tough one. But I will say first of all that I admire you seeking help for your daughter. My parents kind of just shamed me for my ED when I was younger, which of course only fuelled it.
I agree with everything that's being said here. It makes me sad that young people have to go through this, but when I think back to when I was ten and eleven (and even younger than that) girls were already on diets, already trying to lose weight. I think our society is just sick when it comes to the standards it imposes, on anyone, not just women.
You're right that it doesn't help that she's a dancer. That can be one of the worst environments for a young girl because there will be pressure all around to 'lose weight' and stay at a 'low weight' especially if she wants to go professional. Hopefully you can help her build some strength and discernment so that she can take parts of it (ie. keep her body healthy rather than lose weight) and leave the rest.
I so hope it works out and I'd love to see updates as to how she's doing. I really, really, really hope it doesn't escalate for her.