I started to respond to your question, but then realized you didn't clarify whether the mother was slapping an adult, a teenager, a preteen, youngster or toddler. Furthermore, we don't know where the slap occurred. Was the person slapped in the face, in the head, on the arm, on the thigh or on the buttocks? The reason I'm asking for clarification is because each scenario will illicit a different response. I'm not nitpicking, rather I'm just wondering the best way to answer.
For instance, if my mother slapped me on the knee or rear and I was an adult and she did it in a parental kidding manner, then I would say "no harm, no foul." If my mother slapped me in the face as an adult, I would be forced to look deep inside of myself to determine what I did that was so egregious to cause such a reaction.
I grew up in an era where corporal punishment was doled out as commonly as drinking a glass of milk. Children quickly learned their boundaries and the difference between right and wrong. We didn't dare question authority and it was a time for children to have what I refer to as a "healthy respect" for authority and their parents. Did people in my age group question their parents? Oh, you bet we did, but we also learned rather quickly who was the adult and instructor in life and who was the child comprehending the correct way to handle circumstances in our lives. There were no "feel good about yourself" discussions. We clearly saw the lines in the sand and what consequences would befall us with bad behavior. You see, we didn't have TVs in our bedrooms, computers, i phones, i pads, kindles, and so on and so forth. What we had was playing outside with our friends. If we were slapped and lost our privileges to play outside with our friends, then basically, we lost everything. My point is this...we didn't question authority...we didn't dare. When a parent told us to jump, we asked, "how high?"
I'm in my 6th generation and you know what? By and large people in my age group turned out okay.