Well, she's young, but if you're willing, she could babysit at your own house to start, so you're there if she's in a crisis, but you could go do something fun, like read a book in your bedroom, so you're not 'helping' her, and she can keep the money.
You could start that way, so you can observe how responsible she is, and she's doing the work, making the money, but other parents might feel better leaving their kids, knowing you'll be there as well.
I'm not a parent yet, but I wouldn't leave my child, any age, w/ an 11 yr old, unsupervised. I wouldn't leave an 11 yr old *by herself*, unsupervised, for any length of time.