Monster mission - Eva Ibbotson

'We must kidnap some children,' announced Aunt Etta. 'Young, strong ones. It will be dangerous, but it must be done.' Three children are stolen and taken to a bizarre island, which is home to some extraordinary creatures - including mermaids, selkies and the legendary kraken. The island is the base for a very mysterious mission. But the magical adventure is cut short, when the island is suddenly under siege. Can they save themselves and their new friends? 'Funny, gripping, charming and completely irresistible' Amanda Craig