I'd like to preface this statement by saying that I'm not trying to defend the actions of a certain poster.
That being said, why do people's children come up in this? More than racism and sexism, attacking one's child is attacking an invisible person. It is someone who isn't on the site and someone that we usually don't know actually know if they exist or not (i.e. I could claim to have a child and not have one). Furthermore if we can't attack someone's child, then why isn't there a blanket family member flaming policy.
This is somewhere between a 'why' question and a bar rules inquiry.
I think this is the situation with children. People are far more protective of their children, than they are of their parents, because people see their children at their most defenseless, while their parents have always been strong and individual. Thus, when talking about "Your momma" jokes, I believe they are more acceptable, because of the subconscious view on the stability of parents.
As far as children go, because many of them are/were defenseless, the parent of the child feels the responsibility to protect their children. So when someone flames the "invisible" child, by a state of transference, they are, in a sense, flaming the parent of that child as well. Which then falls under the "flaming" section of the rules.
As far as insulting another person's child...is there really a need for it? Are posters on here so immature they can't contain their retorts to the poster they are addressing. The way I see it, in the Bar Room, if someone makes a comment about another poster's child, and is ask/told to stop by the second poster or a mod, then they need to stop. If they continue, they are opening themselves up for infraction/banning based upon the "moderator's discretion" clause in the Bar Room rules.
That's how I see it, at least.