Janice, what is getting me right off the bat is why she thought she had the right to ask you in the first place if you have children. Was that question relevant to your transaction?
No matter; it was inappropriate and I would have said so. Had she persisted I would have asked "Do you or do you not want my business?" If that didn't stop her I would have curtly wished her good day and walked out, leaving potential purchases behind. Followed by a scathing review on Yelp.
I've never been on the receiving end of this from a sales clerk but some friends have pulled the-wrong-people-are-reproducing argument on me. One even said "Look, I know you hate kids but you're intelligent and you should have one for an intelligent couple who wants kids to raise." I asked if she was high. Never heard that again.