- They "forgot" they had a gun. Firearms are deadly weapons. As such, you need to know where they are at all times. If you don't want this responsibility, sell the guns to someone who is more responsible.
- They did not educate their child about firearms. I never leave a child, unsupervised, with any kind of weapon. However, if you own a firearm (and even if you don't because you never know when your child may come upon a gun so they need to know what to do), you need to educate your child about proper firearms handling and use. It's kind of like if you live near any type of water (swimming pool, lake, river, etc)--common sense says that you need to teach your child to swim even though you never intend to let them swim unsupervised because with kids you never know what situation they may find themselves in.
- They "hid" their firearm in a box. Unless your firearm is in your immediate possession, it needs to be secured. Which means that kids and anyone else in your home cannot pick it up and shoot themselves or anyone else with it. Secured means, um secure. Duh.
- They never used the firearm or checked on it. Responsible firearms owners know where their firearms are at all times. You shouldn't have to guess where you left it and by all means, if you own a firearm for any reason (defense, hunting, target shooting) you need to use it regularly. If not, you will pull it out in a defensive situation and either hit someone other than your intended target or said intended target will take it away and use it on YOU. You also need to check on your firearms regularly to make sure they haven't disappeared. Firearms are more often stolen by family, friends, and household help then they are lost to a burglar.
I don't have to guess how this event will impact the family. I have seen way too many parents of dead kids. No matter how the child died, even if by natural causes, the parents think they could have and should have done something to save their child. When a child is lost to a senseless, stupid accident due to the parent's negligence, I have no doubt that these people will blame themselves thoroughly and forever. Fortunately their child is still alive. Many parents aren't that lucky in this situation and what follows for these people is deep depression, divorce, job loss, PTSD, sometimes suicide...and that's just within the first couple of years of a child's death.