The Ultimate Dinopedia (US$5.99) from National Geographic is a wonderful reference app that will quickly become the go-to source for kids who want to find out everything about dinosaurs. There are other dinosaur apps in the store, but this is by far the most complete and expansive. It started as a hardcover book last year and has since been turned into a terrific iPad app for kids four and older.
The app is broken down into three parts: Discovering Dinosaurs, Meat Eaters and Plant Eaters. It's chock full of paintings of many dinosaurs by Franco Tempsesta, and the text was written by "Dino" Don Lessem, just as in the book. (Don even has a dinosaur named after him.)
The app covers about 700 dinosaurs. Each dinosaur comes with a button to play a sound of the correct pronunciation of its name, plus much more information. For many dinosaurs, that's all you get, but others have full-page paintings, Dino Stats (which give you info from the overview page) and a paragraph on the story of the dinosaur, which you can either read yourself or have read to you in a slightly sinister sounding narration paired with nice sound effects. Each of these pages have Fun Facts like "Riojasaurus had only five teeth in the front of its top jaw and 24 more behind them. So chances are it gulped down its food and digested plants in its stomach." There's also Picture Info, such as "Riojasaurus fed in the southern forests of Pangea, the single landmark that stretched across the middle of the earth". Along with this, there are 13 very short CGI videos showing a number of dinosaurs in action. The videos certainly aren't up to the quality of Pixar, but they get the job done. The videos are the only part of the app that works in landscape mode.