I searched and there only seemed to be one discussion on this, and no one seemed to agree.
So I'll ask a bit differently,
1) If your FG isn't your own daughter, are you paying for their dress, or is their parents/someone else?
2) How did you pick their dress? Did you just pick a dress and tell the people to buy it? or did you ask the parents to pick something out for the child?
3) How much do your your flower girl dresses cost?
I'm not sure if I should pick something out and tell my FH brother, "here buy this" or if I should buy it for her, or if I should say, "please buy FH dress #1234 from Davids bridal", or just plain, "please buy her a plain white dress"