Plus, my son is a black belt in karate, and just might react without thinking and defend himself if grabbed! You cannot tell how strong he is by looking.
Isn't a big part of karate discipline and self control so exactly that kind of thing would not happen?
I took Tae Kwon Do, not karate, but this was my experience:
Discipline and self control was definitely emphasized, but it was mostly about control of our actual fighting (i.e. the strikes and blocks themselves, controlled body motions, etc.) and about conflict avoidance. When sparring and practicing, our strikes were supposed to be well controlled--there was no excuse for throwing a strike so hard or fast that you couldn't control where and how it landed, and be able to "pull" a strike that went wrong (either abort or drastically reduce the force before it hit). We were also taught conflict avoidance methods and that in real-life conflicts, getting into a physical fight should be a last resort. However, once someone physically attacked us, it was considered that the physical conflict had already been started and our task shifted to doing whatever we needed to to end that conflict and get away without getting hurt--hurting our attacker was no longer a concern.
IMO, the issue with being grabbed in a haunted house is one of threat assessment, not self control. Looking at it from the outside, we know that a haunted house actor is not a threat because they are not really "attacking" the patron. However, the point of a haunted house is to scare the patrons, and the reason grabbing works to scare is that the patron's brain perceives it as a threat/attack, in an already tense and scary situation*. IME, nothing in a martial artist's "self control" training teaches them not to respond forcefully to a genuine physical attack--quite the opposite. In a real attack situation, your "control" is intended to make sure you can hurt your attacker effectively, so they don't get a chance to hurt you. The issue in a haunted house isn't in how you (general) respond to an attack, it's remembering that you aren't truly being "attacked," despite what all your instincts are telling you.
*Imagine being lightly tapped on the shoulder by a stranger in a well-lit, populated supermarket versus in a dark, deserted alley in the rough part of town--is your brain going to perceive both as equally non-threatening?