IIRC there have been some similar occurrences in other States which didn't result in any adverse actions against the parents. But my memory is sometimes hazy. If I have a chance to do some research later, I'll post about it.
But in any case, I agree with pax. This seems to have been a proper and appropriate use of lethal force. Those "keep loaded guns away from children" laws I've seen create liability only when the child then does something wrong with the gun.
And based on the newspaper account, it looks like the kid correctly did the sorts of things we talk about often: get innocents to a place of safety; arm yourself; and take a defensible position.
"Teach your children well..."
"It is long been a principle of ours that one is no more armed because he has possession of a firearm than he is a musician because he owns a piano. There is no point in having a gun if you are not capable of using it skillfully." -- Jeff Cooper