The name for this is Schadenfreude, which translates to "Harm joy."

Kids as young as 2 years old experience it, and one of the main causes of it is our sense of Justice. We like to see someone who is acting like an idiot fall and bonk their heads or something... But historically we used to draw the line at laughing at someone who is getting more harm than he/she deserved. Over time the line has clearly moved.