The Secret Island (Secret Series, #1)

Four runaways, Mike, Peggy, Nora and Jack, find a secret hiding place—a deserted island on a lovely lake. They build a willow-tree house, make their beds of heather and bracken, and grow their own vegetables. And Jack even manages to bring his cow, Daisy, and some hens to the island for fresh milk and eggs every day! But one day invaders come to the secret island...