Nudging is an essential part of playing pinball. Even if they get a tilt. The machines are more than built to handle it. You shouldn't get mad at all, you should be glad they are getting into it.

Abuse of an arcade device is easy to spot, kicked in coin doors, lifting and dropping the machine. Heavy heavy shaking that makes the backbox sway,

Anything else is fine, You should be more worried about your floors or walls if the play gets very spirited.