The Enid Blyton Secret Series

Peggy, Mike and Laura are staying with their Nan while their dad is off exploring with a photographer named Charlotte. Their Nan dies so they are sent off to live with their horrid aunt and uncle where they have to do lots of chores.

The children, reunited with their parents again (and Jack now adopted as part of the family), go on holiday at the rugged Cornish coast with a family friend, Miss Dimity, and get drawn into a kidnapping case as they discover a boy being held prisoner in a castle tower nearby.