Borrow, Borrow, Borrow!
Grab a pad of paper and a pen. Now, sketch from memory what the flower girl's dress at any of your friends' weddings has looked like. Obviously, it was white … and probably long … and … well, that's all you remember, right?
People's senses go into major overload at weddings, so it's difficult to retain the many details of each event, like the neckline or embellishments on your little gal's gown. Solicit your friends and family for a gently used, yet still adorable, flower girl gown. If anyone has the nerve to criticize your thriftiness, insist that using your best friend's/sister's/second cousin's flower girl gown is a sentimental gesture to the importance of family and tradition and that you intend to preserve it for generations.
Then, buy a cocktail for yourself and a Shirley Temple for the flower girl with the money you saved.