In the US, the questions are not per se a violation of the law, but you are treading on very thin ice because using the answers to such questions in a way that has a disparate impact on anyone because of their gender, age, family status, religion, etc is a violation of the law, even if you did not intend to discriminate against people on that basis.

Just out of curiosity, does it matter what the questions are actually about? Even in the US if you ask a woman if she plans on getting pregnant you're going to have issues. But I would imagine if a woman was answering a general "tell me about yourself" type question and mentioned her kids, the interviewer might ask a follow up, particularly if they have a similar interest or something (obviously staying away from "so are you going to have more?" or whatever).

I'm a guy so I've never been asked about kids until after being hired, and even then just conversationally at lunch.