So, you're basically never going to address the question about if the parents were negligent in how they stored their firearm?
You made a big deal about people being punished for other people's actions but when I ask you only about the parent's actions, you keep avoiding the question. Here's how I'm going to interpret it:
You are refusing to address whether or not the parents were negligent in how they stored their firearm because you know that they did not store the weapon properly. You also know that if you say the parents were negligent, my next question is going to be about whether or not they should be punished specifically for their negligence in how they stored their firearm. Since neither question is related to what a 3rd party did with the firearm, it would undercut the idea that they are being punished for someone else's actions. Why? Because the storage of the firearm is entirely under the firearm owners control.
So, you're seeing the potential problem. If you say they shouldn't be punished for negligence in how they stored the firearm, you'd be saying that they don't have to follow the law. And if you say that they should be punished for negligence in how they stored the firearm then you open the door to what is appropriate punishment in such a case and criminal negligence would make them liable for foreseeable dangers that they disregarded.
Either way, you've realized that they're not being punished for the son's actions but for their own and you don't want to acknowledge that.
I say good day sir.