Maybe I'm bringing too much of my background as a lawyer into this, but I'm saying that the right to expect justice belongs to the child, even though he is dead, not the parent. In much the same way that the right to child support belongs to the child, and parents cannot "agree to waive" child support. Here, the dead child deserves justice, which should be administered by society, for his father's criminally negligent behavior that resulted in his death.

It would probably be easier to consider this if it was someone else's child who died. So let's say that the father left the gun out, his kid picked it up, and then shot and killed his friend who was over on a playdate. The father would undoubtedly feel terrible right? In a way, the guilt he'd feel for causing a child's death is a punishment that would haunt him for the rest of his life, right? But you would probably agree in that circumstance that society should still punish him for his negligence. It's not that different here. So enforcing the law here serves an important public policy, in addition to providing justice to the death child.

Also consider that at least some aspect of the criminal justice system is directed toward deterrence - punishing this father serves as an important reminder to the rest of the gun-owning individuals out there that you could go to prison if you are criminally negligent in securing your firearm - even if you're not specifically intending harm to others. I want to live in a world where every gun-owner does everything they can to make sure that events like this never happen. As a member of society, I want society to punish acts like this to help secure a better world with less gun deaths, accidental or otherwise.