In criminal law, a criminal intent (or mens rea) is typically required for criminal liability.
A boy in Arizona recently killed his father and a boarder with a rifle. |AP|CNN|
First, police don't press charges, only a prosecutor does. Second, I wonder if an eight-year-old could possess the mental state necessary for a double murder.
People UK has more information here including some other recent murders/accidents involving children:
|Boy of 8 murders dad & pal - People UK|
Weapons are inherently dangerous and firearms are incredibly lethal, danger is their utility and their threat.
Children, on the other hand, are typically benign but bear watching.
The lesson I take away from this most recent story is that (if you own guns) it's smart to have a gun lock or gun safe and keep your guns (and/or ammo) there.