Five Run Away Together by Enid Blyton

Unhappy with a strict, unpleasant caretaker, four resourceful children and their loyal dog run away to a lonely house on a nearby island, where they uncover evidence that someone is being kept against their will and that suspicious characters are using the place for secretive, dangerous purposes; by working together, using courage and cleverness, they shelter the victim, gather proof, outwit the wrongdoers and bring the situation to light, turning a scary situation into a successful rescue.