how about basting spray that will become permanant? that might hold something on the back of the leotard. best to try a small section.
what pattern does the mother want? if you appliqued a fabric dancer and put the leotards on it on a large center block, you could just add borders of varying widths and patterns around it and make a neat quilt.
maybe you could use a photo of the little girl in her ballet outfit.
i'll keep watching to see what happens when you try the lycra.