Actually, the age issue of the 7 year old isn't all that salient to whether this was murder or not. There hasn't been any claim of personal threat so that aspect of the discussion is moot. The law does not stipulate an age range at which lethal force can be used against another in the protection of property, or in self defense for that matter.
Had the mother or father died instead of the child, we would have the exact same situation here in terms of legal issues.
Having a 7 year old and a 9 year old, I can tell you emphatically that a 7 year old can provide a very real threat when it comes to the destruction of property. It is amazing what a kid with a hammer can do.