Talk to the kid. Something close to this: "Hi there kiddo. Since your dad doesn't seem to know his ass from his elbow, I'll explain some basic stuff to you for future reference. You don't ever sit on someone's motorcycle without asking first. Motorcyclists like their machines, so don't drip gooey stuff all over them. Finally, if you do either of these things, you should expect to get an ass kicking. Okay?"