It isn't child neglect. I don't agree with this post and please don't presume to speak for me in the future.
It is very common for children this age to own and use guns. The whole world isn't raised in an apartment complex.
In MN childen as young as 10 yr old may legally hunt with a parent present. Obviously, a kid doesn't turn 10, then get a gun. I would guess a MN kid has been firing (or owning) since age 8.
No state permit is required to possess a shotgun, rifle or handgun in AZ, although tt is unlawful to sell or give to a minor, without written consent of the minor’s parent or legal guardian, a firearm or ammunition. We can assume the grandmother gave the gun to her son who then gave it to the boy. Nothing the father did was illegal.