It's difficult to make children understand complex things, and it's horrifying to try explaining something heavy like sexual assault. .. and I've found myself saying 'wrong' and technically not accurate things to my own nieces and nephews because they just simply didn't understand or have the mental capacity for some scenarios and situations. A child needs something concrete to grasp onto, they lack abstract thinking.
The best answer definitely was, "because violence is wrong." But how do you synthesize that without overstepping on the toes of the parents, which allow wrestling, and some punching and kicking between father and son, etc? It was just.. more effective in the short term to tell them, "You don't hit her because she's a girl." And children are more flexible than adults.. so it's easy for them to say, "Oh, Aunt Kyuuei isn't a girl because I can wrestle with her and it's cool. My baby sister is a girl and she's NOT cool with my punch wrestling." It covered all the bases without frustrating the child and making them understand in a way that was actually implemented immediately. I can definitely see it being valid even if it isn't the smartest most bestest answer.