A flower girl is usually a young girl between the ages of 4 and 8 who has a special connection to the bride and groom. Perhaps she is a niece or cousin, or a family friend. She follows behind the bridesmaids with a basket of flowers, scattering them down the aisle.
For some of you, the choice will be obvious. Others may have to worry about choosing between several possible candidates. There's no reason why you can't have two or even three flower girls. It will add to their confidence and ups the cuteness quotient. Avoid choosing someone who is too young. She will be confused and perhaps scared. An older girl may be more suited for a job such as reader, or junior bridesmaid. If your only choices are unsuitable or not that close to you, there is no reason why you have to have a flower girl.
Often, a flower girl is dressed in a mini-version of the bride's dress, or a white dress with a sash that matches the bridesmaid's dresses. But you can choose any sweet dress that ties into the look of the wedding. Make sure it's comfortable and not too long, so that she won't be pulling at her dress or tripping down the aisle. If your flower girl doesn't like dresses, choose a pants outfit with the same level of formality as the bridesmaid dresses.