By knocking them unconscious.
He cannot knock out every threatening being.
For how long? No one is that "angelic".
Perhaps children should learn that no one is that one-dimensionally good and that we always have vigilant about our own personal nature.
True, but you have some who have placed Superman in the Daddy/Santa role, so he cannot be a complex character to any degree--he's a cartoony monument for behavior that cannot work in superhero comics, the very reason DC's editors discarded the Daddy/Santa version (from the mid-Golden to late Silver Age) to normalize him and his relationships to other characters in the DC universe, instead of him seeming like the golden image others were merely sharing panels with, but he (Superman) was barely, believably interacting with the increasingly complex superheroes of the late 1960s-forward.
And considering that comic books and comic book movies/TV is also popular with adults, I don't see why Superman always has to be portrayed as a character for kids.
Initially, he was not in the comics; his creators presented him as a Depression-era vigilante who judged and killed a number of villains when necessary, and did not cry about his actions, as the behavior was reflective of a good number among Americans at the time where crime was concerned (and we must remember that Superman was wildly popular with adults as well as children of this early period, so the idea that Superman was intended to be exclusively kiddie-fare is not at all true). Weisinger stripped that from the character, turning him into the aforementioned Daddy/Santa, which ran its course once comic readership matured in the 60s, with fans expecting their heroes to mirror (as in the late 30s / early 40s) the emotions and interactions of their time--which was not that silly, kiddie version.