The Famous Five: Five Run Away Together by Enid Blyton.

Summer holidays again, at Kirrin Cottage – and the Five are together again! Could anything possibly be better? And when the children become aware of some mysterious signals from a boat at sea, excitement and curiosity descends! The children suspect it’s smugglers, but then they hear a child scream… During Aunt Fanny and Uncle Quentin’s absence (following an unexpected illness), the children are left under supervision of the new cook—Mrs. Stick and her son Edgar, and as a result of their spiteful and unpleasant ways, the five run away to Kirrin Island, and find themselves in the middle of a breath-taking adventure!