Let your kid go through mazes of numbers and collect diamonds. Fire rockets against the diamond eating snake and collect coins in space with your space ship.
And learn simple math all through the adventure.
While going through the levels, your kid will collect diamonds. Personally, I let them have some small price, when enough diamonds are collected. In the administration mode, the diamonds can be reset, and the levels can be restricted.
Apple makes it easy to lock an app, so it requires a code to use the iPad for something else. Go to settings->General->Accessibility->Guided Access and set up a passcode. It is furthermore convenient to set up a short cut, so one enters the guided access by triple clicking the home button
NOTE:
The app works best on larger screens, but it will also run on the iPhone 4s, but requires tiny fingers.