If you are a designer, and you have never watched a child try to use your app, you really should do so. Even if it’s not necessarily meant for kids, it’s still worthwhile. It is a humbling experience. It’s also illuminating. Kids go through the exact same process that adults do when presented with a new UI to learn … they just do it faster and with more emotion.
Where an adult might confront a new Facebook privacy settings page, become confused and annoyed and post some sort of half hearted “y u change, FB!?!” on their wall, a child will try to use the game they love, fail, have a screaming tantrum (and you’ll have to intervene quickly to prevent the iPad from being hurled across the room), but minutes later be happy and using it again. Kids learn things faster than you would ever believe is possible. My son mastered the new YouTube app interface faster than I did. And I do this sort of thing for a living.