The Things have been going absolutely crazy over DragonBox lately. Its a cute little app that introduces basic algebra ideas through a game where you complete levels to make your dragon grow. It starts out using cards with daytime and nighttime pictures on them and slowly changes to positive and negative numbers and letters. When you finish all of the levels you can go back to the bonus round and redo all of them without the picture cards.
Thing 3 finished the bonus levels today and asked for the second app. It was actually a great price on the Amazon Appstore. $9.99 (or 999 coins) and you get 300 coins back which I used to get Thing 1 a reading app.
As soon as Thing 2 saw the next app she switched from it because the first one was "boring." Now she's happily growing her dragons.