Great sale going on right now with Britannica Kids apps (normally $4.99) for $0.99. These are high quality learning apps filled with facts and HD quality images. There are learning activities and quizzes as well to extend the learning. These apps are aimed at ages 6 and up for the elementary student. At $0.99, these are well worth the price. My 3yo is over the moon literally for the solar system app.
These are the additional apps available: Rainforests, Dinosaurs, Snakes, Volcanoes, Ancient Egypt, Aztec Empire, Knights & Castles, and Ancient Rome.