I dont' know if there's an official policy. I don't think it happens often.
I kind of question why someone would want to join a sorority if they had a child. I can understand why a person in general would want to be in one. But if you have a child, shouldn't there be different priorities in your life? Once you have a kid, they should be your number one priority above all. A sorority is a large time commitment.
Also if they're married or still have a relationship with their baby's father, what about mixers and such? What if the woman started hooking up with a guy? I'd feel so overwhelmingly guilty for possibly leading to the cause of a dissolve of a marriage. I understand this can happen all the time with girls cheating on their guys, but once there's a child involved it's a much bigger deal.
I'll probably get flamed for some of this, but keep in mind that it's just my personal opinion and that we're all entitled to our own opinion. And i'm only speaking on perspective from an NPC point of view. I've heard of NPHC members who have children and have had a very positive experience. So no one go flipping out, lets try to keep this a civil thread.