Little girls would look good in flapper dresses, since they don't have chests anyway. Ball gowns just look weird on them.

Disney wants Tiana to be a Princess like the other Disney Princesses, and not a servant-girl-who-marries-up like Belle or Cinderella. (Though neither of these characters was poor -- Belle was from a respectable middle class family, and Cinderella's papa was rich.) Personally just hope the (many) re-writes don't completely kill the script and character.