... Then why is it not okay to go and beat them up, even as an adult? Since the difference between adults and children is moot following this logic.
You are a father, or a mother, or an older sibling, whatever. Your kid/younger sibling is bullied in school. Just go and beat the bully up! Beat them to a pulp so they got zero desire to do it again! At the bare minimum your kid will be fine during the bully's hospital time.
If you disagree with this but agrees with the thread title, then explain the justification.