I doubt you would put your children in a padded cell the minute you found out there may be something mentally wrong with them. All it takes is for the neighbor kid to give him a BB gun and him to scare the parents and have them call the cops, when the cop shows up and kills your child was it your fault for having a child that had a disability? Are you expected to have your tubes tied to make sure it doesn't happen again?
I understand what you are saying, I have seen friggin pink AR15s that I swear look so fake I'd be surprised if it even shot marshmallows let alone the high velocity 5.56 round it actually chambers, but saying "If you kid is too dumb to......etc." is a cop out in itself too, kids don't always listen and sometimes no one even knows anything is wrong with their child until they come out one day and he is drowning his little cousin without reason or cause, its disturbing but it happens, blaming the parents isn't the end all of solutions.
I'm sure you have done at least ONE thing in your life you knew you shouldn't have and that your parents would have or did forbid you from doing, if you had died anyway would it have been a case of "Well the kid was just too stupid, glad it happened, he deserved it." because now you are a perfectly functioning and concealed weapon carrying member of society.
What I'm getting at is that human nature is to do AT LEAST one or 2 stupid things, sometimes without explanation, in our lives, that shouldn't fall back onto our parents who forbid us to do those things in the first place or be treated as a "Well he was just too stupid to survive." type of situation.