I too compliment the OP for continued reflection and introspection on how to best handle a situation like this.
I'm concerned about two assertions Hank makes here, however.
1-That lack of SA was the "causal factor" to the incident". No, lack of SA was a contributing factor that allowed touching of the firearm to take place. The proximate cause of the incident was that a third party engaged in battery (unwanted touching) involving a deadly weapon. I don't know if others recall the short-lived and very unpopular PSAs from a few decades ago encouraging people to lock their cars so as to avoid "helping a good boy go bad." I do not blame law abiding citizens for the illegal conduct of others. I do encourage decent people to do what they can to avoid being victimized. And so locking your car door, having good SA, and using a retention holster are good ideas. They are to be encouraged. But at the end of the day, a criminal does not get a pass just because a victim didn't make it really difficult to commit a crime.
Indeed, in the final analysis, I resent the fact that I am practically compelled to lock my home and car, to keep everything of possible value out of sight lest some punk smash a window to grab what turns out to be an empty book bag off the seat, to be on constant alert for those who would harm me, even to carry a firearm to help assure my and my family's safety. We live in an imperfect world, no doubt. And we must deal with that reality. But let's never place blame on the innocent, law-abiding, decent, or civil. Lack of SA, lack of being armed, lack of locks, etc, etc, may allow the criminals, scum, deviants, and ill-mannered to operate more easily. But it never justifies or excuses their conduct. While it can be a fine line, it is a crucial line to remember always the difference between encouraging people to protect themselves from criminal conduct and actually blaming them for the conduct of those criminals.
2-That we "know for a fact" the young person was "harmless". While it is true that all information at this time indicates the young person was not a serious threat, was that obvious at the time of the incident? As others have pointed out, 12 year olds are hardly immune to gang activity and the commission of serious crimes. To judge the OP on his initial reaction based on what did or did not happen much later is to require him to be prescient. A slap n the hand for unwanted, illegal touching is not an infliction of "harm". Whether it is the best way to handle any given case of unwanted touching is a question to be answered on a case by case basis. In this case it seems to have worked out well and had the desired and necessary effect.
That some other course may have been even better does not change that fact. No doubt, the best course is to avoid the problem entirely with good SA and keeping the firearm physically out of reach, using verbal commands to end any attempt to touch before contact is made, etc. But it is good to consider how best to respond if a momentary lapse does allow someone to touch or even grab for a firearm.
Charles