White tea length gowns made of chiffon or satin with a matching wedding color (or white) bow is standard for a flowergirl. However, if white is too plain, than select a gown that resembles the bridesmaids' dresses in color, style and length. Rhonda Allen, Bridal Consultant New Beginnings Weddings
Traditionally the flower girl wears a dress that resembles the brides dress. They will wear a white or ivory dress with a sash or bow in the color of the bridesmaid dresses.
Now a days, flower girls have been wearing all different kinds of dresses. For example dresses that look like Easter dresses or holiday dresses. I would just make sure if she wears a dress in a color and/or pattern it will compliment your bridesmaid dresses. She can really get away with anything, nomatter what she wears she will look adorable.