One of the most colourful and flexible selections for a marriage theme is the employment of butterflies. You can start your butterfly theme with imprints on your invites and carry it thru from your bouquet to your thanks notes.
You can think about using butterflies in bouquets and floral displays, very similar to you will see them in nature. Using butterflies printed on engagement headlines, save the date cards, invites, programs and many thanks notes will set the tone for your butterfly theme for guests. Taking your theme a step further and choosing butterfly formed floating candles for centerpieces, colourful butterfly formed chocolates or candies for table favors and even butterflies made out of sugar made to seem like they're flying from your marriage cake would be great options. One tip when selecting dresses for them, don't select "baby" dresses. This practice is an old custom whereby a girl was dressed in the same robe as the bride and sent marching before the bride. It is trusted in the traditional times that doing this would attract malevolent spirits away from the genuine bride so they can't curse the marriage. With these, you have got something to begin with in finding the best flower girl dresses.