Someone else defended my point so I don't have to but yeah. Maybe I am old fashioned but you do not involve the police in a family dispute unless there is no other choice. They are on par with nuclear weapons in the 'family dispute' circles, only to be used when there is no other option available in truly violent situations. I can think of many situations that would be a good idea to involve the police in, all of them involving abuse between adults or abuse by an adult onto a child.
In the case a kid picking up a knife and threatening? Yeah might be violent, but doubtful jumping straight to the police is going to help anyone. Lots of other options to be explored first, if you involve the law at all.
There are a huge number of things you can do to disarm such a situation and then find a way to curb such behaviour, all without involving the police, trials and years behind bars. Something no kid should have to go through for a single moment lacking of impulse control unless they are proven a content, and obvious, threat to society as a whole. Defiantly they don't deserve it if, outside of that situation, they are quite reasonable and no where near as aggressive. Circumstances can easily make a kid over-react in the worse possible way, after all.
My personal experience on this: I was the one holding the knife.
The bastard deserved it but, yes, the whole situation was disarmed by my younger brother, who had more smarts in dealing with such a situation then a younger brother should. Besides if they had involved the police it would of been there AFTER the fact, after it had already de-escalated to the point the only thing the police could do was file paperwork and set a trial date. How would calling them have helped that moment, or at all?
Hell, we didn't even involve any parental units and it was all worked out, more or less, without blood shed. I say more or less because the person I threatened still is a dick but now technically he is family by marriage.