Flower girl dresses should be cute and fun, just like the girl who's wearing them. If you're having trouble finding the perfect look, take a cue from Hailee Steinfeld, actress in True Grit, who wore a little white dress at the Day of Indulgence.
Hailee's white lace garment was simple, girly and adorable. The ribbon straps tied over the shoulders for a little flair, and the hem hit at mid-calf. The bodice consisted of horizontal pieces of lace, and the sheer skirt ended in a pretty flowery design at the bottom.
She wore white peep-toed platforms with cork bottoms for a summer wedding vibe and kept her makeup minimal with a little bit of eyeliner and matte powder. She wore her long hair down and straight for a fuss-free style. No jewels were necessary for this understated look.
Something similar would definitely work for a flower girl, no matter how old she is. A two-year-old or a 15-year old would look equally dolled up in a lacy white dress and white sandals. Just find a shape that is age appropriate, and she'll be ready to toss those flowers all day!