I made 9 flower girl dresses for my sisters Aug wedding a few years ago. The three smallest flower girls were all under a year old and I used a sundress pattern that I already had. The 6 older girls ranged in ages from 2yr to 8yr and I used a multi-sized pattern. I also helped my sis pick a pattern that would be easy to whip up. They were sundresses in a bright pink/orange ombre. Instead of making a sash for the dress out of the fabric like the pattern called for, I used a wide grosgrain ribbon. Looked great and saved time! Good luck!
And really, when it comes down to it, you may be able to find something cute and fairly cheap at Target or Walmart and save yourself the stress....