It's totally fun to be an adult and dress up for
MNSSHP. Your mermaid outfits sound so cute. And you won't be alone, there is a good mix of regular dressed, half costumed, and all out costumes by adults.
I have two boys so it's totally different, the first year we did a pirate theme as my little one wanted to be Jake (and the neverland pirates). So my older son wore a pirates of the Caribbean tee and hat and I wore a generic pirate girl outfit.
Then last year my boys were ninja turtles (

I know not Disney, I was so embarrassed, lol

but they got so many compliments even from CM's

- my older son didn't wear it the whole night because he didn't like all the attention and it was sooooo hot) and I just made a mickey pumpkin shirt to wear.
all the outfits you have planned for the girls look so cute.