Edith Nesbit 
Story of the Amulet 

While the childrens mother and father are out of the country the children are staying with the "old nurse" in her boarding house. There is only one other boarder, an old Egyptoligist, whom the children soon befriend. They learn of an amulet that has the ability to grant their hearts desire, and soon buy it. After making the purchase, they learn that it is the only surviving half of the amulet. Though the half of the amulet cannot grant their hearts desire, it can serve as a magic portal permitting time travel. In this book, the five children, Cyril, Anthea, Robert, Jane and the Lamb conclude their trilogy of adventures.