The Witch's Daughter
Our three child protagonists are Perdita, the so-called witch's daughter, a half-wild girl who is teased and bullied by the local kids; her greatest dream is to be allowed to go to school. Brother and sister Janey and Tim are visitors to the island. Imaginative, sensitive Tim is desperate for his father's approval; Janey is blind, and keen for more independence than her anxious parents will allow. Janey is particularly interesting, because her special strengths shine during the adventure -- she is extremely observant, with an excellent memory, and (spoiler) at the crisis of the story, it's Janey who saves the others by leading them out of a pitch-black cave.
This is a beautifully written novel for younger readers, but alas, I fear they might find it a little slow to get going. I think I find these old books so satisfying because they don't talk down to their readers, and the quality of the writing is wonderful.