The locker example is very confusing. I suggest teaching them how to do trusted coin flip over chat. One kid chooses 0 or 1 and any salt, then hashes them together and sends the hash to the other kid. That kid chooses 0 or 1 and publishes his choice. The first kid publishes what he hashed. The second kid verifies the hash. Both proceed with XOR of two choices as the final outcome. Make them actually do it with GtkHash or something (just don't use md5, it's not secure enough against the kids these days).