Leaving Grandma, who scorns dinosaurs, drowsing on a museum bench, Katie slips through a forbidden door for a prehistoric adventure: she helps lost baby Hadrosaurus find his parents. Meanwhile, the two see a lot of other dinosaurs, all properly identified, and Katie escapes from a Tyrannosaur by tossing it her lunch. The plot's predictable if ever-popular, while Mayhew's creatures are appealingly anthropomorphized and set in a lush landscape, skillfully rendered in sepia line and glowing watercolors. (Picture book. 4-8)