Murphy's Law. Anyone around firearms in use is at some risk of serious injury or death, regardless of safety protocols. (I remember on one of the two days I spent time at a firing range with my revolver, there was a nearby ricochet.) The club was formed in 1949, and I'd be curious to know how many children have fired weapons there since, even automatic, and how many other such fatalities have happened.
This is definitely the parents' responsibility, but I don't see it as a crime. It is a consequence that carries its own inescapable punishment. Jail on top of the destroyed private lives would be an unconscionable injustice.