What do you mean "letting children insult one another"? it seems to imply parents / educators have complete control of children's interactions, and this is simply not true.
That said, there should still be intervention; verbal abuse (which is what it's called when adults do it to one another) should not be normalised, and "deal with it" / "grow thick skin" isn't it either. Because they will take that "thick skin" with them in e.g. an adult relationship and use that normalised verbal abuse on their partner, kids, people they work with, people they interact with on the daily, who did not have that same (abusive) upbringing and thus are harmed by the abuse.
TL;DR, kids will be kids, but don't raise them thinking verbal abuse is OK and that it's the victim's fault for not having thicker skin. "enable the children to learn to deal with such comments" sounds like victim blaming to me.
That said, there should still be intervention; verbal abuse (which is what it's called when adults do it to one another) should not be normalised, and "deal with it" / "grow thick skin" isn't it either. Because they will take that "thick skin" with them in e.g. an adult relationship and use that normalised verbal abuse on their partner, kids, people they work with, people they interact with on the daily, who did not have that same (abusive) upbringing and thus are harmed by the abuse.
TL;DR, kids will be kids, but don't raise them thinking verbal abuse is OK and that it's the victim's fault for not having thicker skin. "enable the children to learn to deal with such comments" sounds like victim blaming to me.