Copycat

Show a video capture of an action from your app, game, or product and ask,  “Can you see if it’s possible to make what you just saw happen? Sometimes it might be possible and sometimes it might not be, so if you think it’s not possible, just tell me.”

Why this technique
Having kids replicate something specific will allow you to see which parts of the interface are easy to understand, and which are confusing. Even when your example is possible to replicate, telling them that there’s a chance that it’s not possible can ease frustrations and make it feel like less of a test of their abilities.