Miss Peggy Post, etiquette advisor extraordinaire says this about flower girl etiquette, "A flower girl traditionally wears a white or pastel dress of midcalf length, white socks and party shoes like Mary Janes. The dress may be similar to the bridesmaid's gowns but should be appropriate for a young child. Headdresses include wreathes of artificial flowers or ribbons or flowers braided in the child's hair...flower girls carry a small bouquet or a basket of flowers, but as a rule, they no longer scatter petals before the bride." (Note from Melissa: I've only seen flower girls do this and it's delightful every time- if you like tossing flowers than I'd recommend it.)
Miss Peggy also wrote, "Flower girls are usually between the ages of three and seven. Flower girls are often young relatives of the bride or groom. There may be more than one flower girl, and this is a nice way to include girls too young to be junior bridesmaids. Their role is to proceed the bride down the aisle, carrying flowers or flower baskets...the flower girls stands with the bridesmaids during the service, though young children may sit with their parents or the bride's family...in the recessional, the flower girl walks with the ring bearer (if there is one), directly behind the couple.