Monster Mission- Eva Ibbotson

Three children have been kidnapped by some mad aunts and panic is spreading. But Myrtle, Coral and Etta are far from killers - they need the children to help them in a caring mission. The secret island where they live is a sanctuary for wonderful creatures - mermaids, selkies, the boobrie bird, and even the Kraken. The kidnapped children love their magical life - but the Island of Monsters has become the target of some dangerous villians…