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.
Tada!
And evidently if you press her belly button she sings like Ariel.
She's worn it all night last night, slept in it and worn it all day today even though it's 60 degrees in the house. Had a bit of an argument about how human mermaids don't wear their fins and shells into the bathtub though. :)
I love it that kids are so happy with such rudimentary things.