Without reading the linked story I can say that most likely useing deadly force in self defense against those kids (or at lest the one who pointed the gun) would be ruled as lawful. We are not required to distinguish betweem real firearms and toys/replicas. Generally the rule is what a "reasonable man" in that situation would have thought they were faced with.
Some of us have experience (recent or ancient history - "before you were born, youngster") with kids coming up and doing harm. And then there are also the feral yutes out there.
"Be polite to everyone you meet - and have a plan of how to kill them" is not just a cutesy saying.