I think that entirely depends on how hard a child is being hit. A smack across the back of the head is embarrassing, but that's about it. Wailing on your kids until they can't walk straight for a week is *incredibly* damaging.

My mom pretty much never controlled how hard she hit. She left me and my sister with welts and bruises whenever she raised her hand. And yes, she would leave me unable to walk for a week after her beatings. It took me until my mid twenties to stop flinching whenever *anyone* would raise a hand around me (for innocent shit like a high five or to wave hello). That was entirely because my mom hit me way too hard growing up.

You and your friend are thinking in black and white too much. There are different degrees of hitting. Some is discipline and some of just pure malice.