My worst was a 4 yr old from hell.
The captain greeted the little ******, and then we continued chatting. A few minutes later, the plane, a boeing 737, yawed quite agressively left and right. He was playing with the rudder. We then told the little mongrel to keep his hands to himself or he would get the boot. After a few more minutes, he had decided that the start levers were his next target. He reached for them...............had one in each hand..............and then he got the boot.
I like to think I learn from my mistakes. No unaccompanied minors on my flight deck.