Choosing the right dress for the flower girl is important. This special little lady gets to be a princess for a day. For one day, she gets to have the focus on herself (even if she doesn't understand precisely what is going on in the ceremony). Most little girls will tell you they love the idea of dressing up and being as pretty as a doll, but there's one really key factor that will seal the deal with the happiness of the flower girl: if she loves her dress.
The most important aspect of a flower girl's dress is comfort. Little kids have a much lower capacity for being uncomfortable than adults do. If your little girl is wearing a dress that pinches or pulls in the slightest, you can expect to be hearing about it throughout the wedding. Make sure she is wearing something that fits her just right.
In addition, colors are a big deal to kids. (Although they can be a big deal to adults, too). Nonetheless, don't expect your flower girl to be enthusiastic about her dress if the color doesn't work for her. Colors are very symbolic for kids and you should work to find a dress that symbolizes something awesome to her.
Most importantly, the best tip that I have for helping you make sure that your flower girl is happy with her dress is this: make her feel like she picked the dress out on her own. If you give her a handful of dresses to pick from, all of which could work, and let her pick...you'll be doing yourself a favor. She'll feel like she picked the dress on her own and won't want to go back on her word and choice by complaining about the dress. Do this and you'll be set for the day.