Boo-to-You! Heather, and thanks for visiting the Disney Parks Moms Panel.
Mickey's Not-So-Scary Halloween Party is so much fun for Guests of all ages and yes, you will see many families in costume!
You are wise to find out the rules ahead of time so you can avoid setbacks when entering the park. I encourage you to review all of the costume guidelines in detail on the
Mickey's Not-So-Scary Halloween Party page. In short, you'll want to keep your costumes family-friendly and safe. Masks of any kind are not allowed for Guests ages 14 and older, but children may wear some masks as long as eyes are visible and they don't cover the whole face.
I think a Little Mermaid theme sounds awesome and you should definitely be able to achieve this look within the costume guidelines. Remember, you'll also want to be comfortable so simple costumes tend to be more fun and last longer when running around gathering treats all night.
Wishing you a spooktacular visit with your family, Heather! Please let us know if you have any other questions as you plan.
Andrea U.