Island of the Aunts — Eva Ibbotson

Original title: Monster Mission.

WELCOME to the Island of the Aunts!

Aunt Etta, Aunt Coral, and Aunt Myrtle need help caring for the mermaids, selkies, and other creatures who live on their hidden island -- and they know that adults can't be trusted. What the kindly old aunts need are a few sensible and sturdy children who can keep a secret. And while kidnapping is generally not a good idea, sometimes it just has to be done. (After all, some kids just plain need to be kidnapped.) When the newly kidnapped assistant caretakers, Minette and Fabio, arrive on the island, all kinds of amazing and wondrous things start happening.