Liam first said he wanted to be a shark, and I knew where we could get a shark costume, so we tossed around a few ideas with Carolina - Nemo, a squid, mermaid. Mermaid won, mostly because Carolina wasn't set on anything and I pushed it ;)
This costume made possible by my mom, she had the green fabric and made the fin - which was pretty difficult. I just painted shells and a starfish and glued them on. Oh and I made the fishy trick or treat bags.
It was pretty cold - so I layered a sweatshirt under both kids - Liam was quite warm and Carolina didn't realize she was cold. There weren't a ton of houses in our neighborhood open for business - but I think they got just enough candy (which is not so much that I have to eat a bunch of it for them).