I followed this tutorial from Girl Inspired. I even did my first ruffles. It's totally for sewing beginners. If you are looking for an easy mermaid style to impress your little ones. This is the one.
I made this just a little longer so it can drape to the floor.
It's a tight fit, which suits the mermaid tail style. With a stretchy knit, there's still pretty much room for little legs to do some quick little steps. The only problem .... er... I'd say is walking up and down the stairs....:D