I suppose you could tell him that it really should be your choice as you two will be spending quite a bit of time together. Plus, she really is too young. She could be a Jr. Bridesmaid though, so that argument may not hold up.
It is typically best to choose someone that you are close with and is reliable. But, if this is something that he really wants, it is a very small concession. Of course, he could always have her stand on his side. There are no gender issues these days.
12yr old bridesmaid
