Do you have any idea how I can make a fish fin on a little girl's back for a VBS costume?
Perhaps you could make the shape of the fin from a clothes hanger, cover it with material and sew it on the costume.
Several things come to mind...cardboard covered with fabric or paper, fun foam, covered styrofoam, or, as previously mentioned, a wire base covered with fabric or paper. The tricky part will be getting it to stand up straight...maybe attaching tabs along it and reinforcing it with wire (think shelf bracket) on each side? This method would be easier with a solid fin (cardboard, fun foam, or styrofoam) than it would be using wire.
It's vacation bible school, usually during the summer break.