Why would I want cops doing that instead of social workers or teachers doing it?
No one becomes a cop because they want to be nice and help vulnerable people. Some might say they did but that is some coping technique. Being a cop involves exerting violence towards people who are vulnerable and desperate, and to become one you have to be fine with this. Some would say that this alone is enough to deem a person ethically dubious.
Even if one would accept the premise that society requires some degree of organised violence towards its members, one would also have to handle the question of accountability. Reasonably this violence should be accountable in relation to the victims of it, and police institutions inherently are not.
I think that we should also note that the other person above used "childishly" to denote something negative, apparently they don't think of kids as the light of the world and childish as something fun and inspiring. This is something that makes me quite suspicious of their morals.
Maybe you and I have vastly different experience with police. Disclaimer: From a rather small US state.
Your other note is also well taken, it does however not imply that anything a kid or teen does is OK or automatically positive.
Finally, it's OK to be suspicious. I am too. What I am saying is that one cannot just make the decision "all cops are evil or must be treated as such" and then hope for a good outcome in all cases. I argue it's a better policy to keep an open mind and decide on a case by case basis.