Perform daily/weekly tasks to earn points towards your next level. Each level unlocks gear and equipment for your avatar as well as a new title. Don't be your "Average Joe" when you can be a "Global Super Hero" and make a difference.
Designing For Kids Is Not Child's Play – Smashing Magazine This again goes into detail of designing an app, it also gives reference to other apps. Which makes it easy to understand and compare with other apps.