Parental negligence is, however.
An eight hour course to teach people that leaving a gun laying around and in reach of children is a bad idea is unconstitutional, though.
The thing is you could sit through a day long course on gun safety and never would they tell you specifically not to leave a gun with a bullet in the chamber in the hands of an unsupervised 5 year old. Common sense eh - not all people use it at all times.
We were shooting squirrels on grandma's farm with .22's when we were not yet into double digits (loved grannies canned 'meat'). The thing is guns were just a tool for securing food and/or keeping varmints out of the root cellar, and they were treated and used like a tool - a very dangerous tool. Never did we treat guns as if they were some special item with their own inherent rights. They were never a 'weapon' (we had knives for that) nor should they ever be weapons in the hands of civilians.
If we're going to allow people to have weapons to protect themselves from other people, what's the sense in having a whole branch of federal, state, and local government to provide the service for us?
A 'well-armed' militia is the biggest joke of all.