I have made several using this free pattern.
https://www.sewmuchado.com/2016/10/a...e-pattern.html Just print it out at 50% of size for small children and 75% for a bigger kid. Works great and is super easy. I let some of the kids pick their own fabric, I recommend sticking with fleece or minky for the body, but some of the fancy knit metallics work fine for the tail fin part. You can lay the knit on top of a piece of fleece and quilt it for the fin effect.