I think I saw my boss abuse his child at the office. What should I have done?

And yes, do start putting out feelers for jobs elsewhere — partly so fear of losing this job doesn’t stifle your compassionate impulses, and also because your boss, by your own account, is “not pleasant to work for.” Even if he knows he can’t get away with grabbing and shaking employees, he may be engaging in other abusive and bullying behaviors toward you that have damaging mental and emotional effects. That’s not okay, and you deserve better.