Short of someone outright giving the child the gun, that’s the only option left. If a 9 year old can get to a gun, it’s improperly stored no matter where it is.
Other than my personal carry which stays on my person (or a quick open safe in my nightstand when I’m sleeping), the rest of mine are locked in a safe unloaded, with their magazines stored in a different smaller safe.
If my kids were able to get to the guns, they pose no damage. They would need to be able to get into both safes and intentionally load them to pose any threat to anything.
The statistical probability of this happening is probably not zero, however, it is still probably remote in the extreme. Even if it were to happen, however, as someone else pointed out: that would still be considered improper storage, because the point is that nobody but a responsible adult should be able to get to it, especially not children. 
If that's the case probably not charge. I'm sure that's happened at some point. But kids are playing with guns that are just left out in a lot of these cases and for those, yeah, maybe someone should face charges.
Just another murderer armed by just another legal gun owner. It's only noteworthy here because of the extremely young age of the murderer, not because "responsible gun owners" seldom arm people who shouldn't have guns.