Dinosaurs Before Dark by Mary Pope Osborne

Jack and Annie, a brother and sister, discover a mysterious tree house filled with books. When Jack wishes to see real dinosaurs, the tree house magically transports them to the prehistoric past. They explore a world of pteranodons and T. rexes while trying to figure out how to return home. As danger approaches, they must think fast and work together to escape. This is the beginning of their time-traveling adventures.