"Tragic failure of parenting in this case" - glad you have a sense of responcibility; being that i'm working with 12 year olds on a daily basis i can tell you a lot of them don't do exactly what they're told - even if their parents are doing the best job they can.
But families don't exist in a vacumn - there is a neighbourhood, a community beyond that and community policing inside of that. Unfortunately when the police have a shoot first and not have to worry about being indited for murder when they are wrong they have a social system which supports making mistakes like this.
Again, i have never seen a police officer in Ireland carrying a firearm. (have in england on the transport system when there were terror alerts, and it felt really weird) i did get stopped while carrying my granddad's airsoft rifle (no orange tip) and had it confiscated - because they are used to rob people, looked and sounded enough like a gun to fool people; guess what, i wasn't shoot at, because the police felt comfortable approaching me (this was outside the US embassy, and embassy security do carry firearms)
So you can easily blame the family, and pretend that all fault lies with them; or you can look at larger government/societal responcibility; in Ireland we have a paternalistic government, which i happen to detest (but that's cause i believe individuals should be treated like adults not children), but they have taken the responcibility to keep children (and everyone else) safe.
You CAN choose to lay the blame entirely on the parent's shoulders but you will be missing the fact that other societies don't have this kind of problem; and it isn't because of better parenting, it is because of better governance. (and sure, in a democracy you should get the government you deserve, so i'd lay the blame on the people not the will of the people which the government is - at least in this case - representing)