I think she likes her birthday present!
I used McCalls 5498 but sewed it up in all knit and stretch fabrics so I omitted the zipper. I also had to take about 8" off the tail length. I cut the size 3-4 so it is big on her and will probably fit her til her feet start to stick out the bottom! I'm thinking stretch fabrics is the way to go for costumes. The construction is so much quicker, there is room to grow and no zippers to put in. It was mostly constructed on the serger. I modified the bodice by making the little cover up separate and adding my own sea shells to meet her requirements for a "boobie bra". I should have redrafted the fin part to be smaller in proportion to her body but I still think it is cute. I think she is happy with it.