I went to a local thrift store and picked up a bunch of tank tops and children's skirts.
All I had to do was take in each tank top so that they would fit either of my girls. Then simply sew one of the skirts to the bottom of the tank and voila! You have a mermaid dress.
With left over material, I added the "tail" to the bottom of this top.
Once they get over their mermaid phase these dresses can also double as flamenco dresses!!