The Voyage of the Dawn Treader by C.S. Lewis

Lucy and Edmund, with their dreadful cousin Eustace, get magically pulled into a painting of a ship at sea. That ship is the Dawn Treader, and on board is Caspian, King of Narnia. He and his companions, including Reepicheep, the valiant warrior mouse, are searching for seven lost lords of Narnia, and their voyage will take them to the edge of the world. Their adventures include being captured by slave traders, a much-too-close encounter with a dragon, and visits to too many enchanted islands, including the place where any dreams come true, no matter how big... or dangerous...