Most likely, the kid would lose his grip. That still does not get him hit on top of the head, and not with enough residual force to kill him outright with a single blunt force trauma to the skull.
Note that 11.3 ft. per second=7.7 miles per hour. This does not take into account muscle resistance by the kid, the guys hand, or simple gravity. (Many shooters, especially inexperienced pistol shooters, anticipate recoil and actively push the firearm forward and down and throw the shot low and left.)
Kids get hit harder playing baseball.
If this is on the level, it is definitely a freak accident, but IMHO, something stinks.
Table, or not, the gun always goes up-never to the crotch. When the gun has 60 ft-lbs of recoil and it's a handgun, it womps the inexperienced in the head. Novice adults I've seen firing TCs, end up with the things ~6" from their head.
Agreed, it does seem highly unlikely. Even in free recoil, I can't see a handgun, or any gun, inflicting a fatal blow to the head.
A 12 year old is plenty big enough to handle a .454, with some instruction. Repeatedly or accurately, probably not, but a single familiarization shot should'nt be an issue using a good stance and proper grip.
Would you try to hold something that could crush your skull? IOW, if it's going to crush your skull, how would any shooters hands survive repeated firings with minimal trauma?
A .454 kicks plenty hard, but the skull is not that fragile.
Walking into a door at a decent pace would impart considerably more energy to your head.