So as I munched, I inventoried my scrap box in my head and then ran upstairs when I finished. I had a blue t-shirt and an ugly, but very bright fabric skirt I got for free somewhere.
I brought down the blue shirt and cut it while on her to make a "bra" for shells - I will do a much better job on the next one now that I have a pattern, only if I have to make a next one, she's happy enough with it as is. Kind of has a Flintstoney aspect to it at the moment. :)
Then I cinched up the skirt with the elastic band to fit her (It's too long, but we just rolled down the waistband--it'll grow with her if she intends to be a mermaid in the years to come) and with the excess material cut freehand a big fin shape. I zigzag sewed around it to keep it from fraying and then sewed it to the middle back of the skirt.
I love it that kids are so happy with such rudimentary things.