At DefCon 2011 in Las Vegas, we hosted about 35 kids, 8-15 years of age, in an hour-long hands-on workshop about locks and how to analyze them for vulnerabilities. Many figured out not-so-simple attacks. We also used puzzle boxes from Japan with secret sliding panels. Many were able to open them the boxes.
It was the first such presentation at DefCon. Marc Tobias, Tobias Bluzmanis, Matt Fiddler, Kevin Manson, and Tommie Ruth Blackwell participated.