I suggest you educate yourself on the realities of knife injuries.
It takes NO training, skill or strength to kill someone with a knife, a five year old could do it.
Hopefully that kid will think twice next time he decides to threaten people with a weapon.
You do realise that kids, and it is mostly kids, who carry knives also use what are called "glimmers". They carry TWO knives, the "glimmer" is the one they show you, brandish it around, get your attention with it. If a victim, a have a go hero or a police officer try to grab the "glimmer", the kid pulls out the second knife (which you won't see because your attention is on the first one) and stabs you with it.
The kids who carry knives aren't stupid, they didn't just wake up and think to put a knife in their pocket over breakfast. They know the tricks, they know how to distract people, they know how to use those knives.
The officers used Taser because of the threat the kid was holding a "glimmer" while hiding another weapon.
Why do you think people in London and other big cities don't get involved when they see kids commiting robberies or stealing bikes?
They know, from living in those areas, the tactics the kids use.