Yes, don't hit kids. But then again, saying you should just talk to kids at an eye level isn't completely realistic. Kids aren't adults. It sounds like a good thing to say you should try to talk to kids at an equal level to have them learn to be mature, but do you really think an impulsive and immature kid still in kindergarten to take your words as seriously as you make them out to be? Most kids will not learn unless they recognize an outcome or consequence of their mistakes or at least the dept of it. Gently slapping Chiho on the cheek is a sign to show that Sakaguchi is severely disappointed in Chiho for going off on her own and getting lost again. If he just told her off again, she'd either just take it with a grain of salt or forget about it. Kids are forgetful and impulsive. Even if you say Chiho is innocent here, she dismissed Sakaguchi's permission and went off on her own.
Obviously the lesson isn't to imply that you can beat your kids, but physical discipline is an effective way to have a child learn consequence. Verbal punishment doesn't often work on kids, physical punishment can be more impactful. Obviously, don't fucking punch a kid across the face with your full strength, but a light tap to the arm, face, calve, or just a pinch is fair punishment. Even then, at least in this case it's practically just contact and the message was clearly received.