Who picks the dress? Traditionally, the bride-to-be chooses the flower girl's dress, just as she picks the bridesmaids' gowns. However, in some instances, the bride may ask the flower girl's mother to select an appropriate dress, within constraints (such as color or length).
Who pays for the flower girl's dress? Usually, the parents of the flower girl are responsible for buying her dress, although you can certainly offer to help them with the purchase if you are so inclined.
From "how to do things . com"