The Secret Mountain (Children's Book Classic). Enid blyton

Captain and Mrs. Arnold are flying off in their plane, the White Swallow, and apparently have crash-landed in a remote part of Africa. The Arnold children, home from school and in the care of a family friend, Miss Dimity, team up with their friend, Prince Paul of Baronia, and go flying off in his royal plane to Africa to rescue their parents – and get tangled up with a strange and secretive African tribe with flaming red hair and living within secret tunnels and caves inside a mountain.