Answer: Managers frequently forget that every interaction with an applicant is part of the interview process. If they drop their guard during coffee breaks and lunch, they can inadvertently wander into unwise areas of questioning. So your interviewers may simply be making ill-advised small talk.
Even if this is a devious screening tactic, ducking the question would be difficult. If you point out the inappropriateness of the inquiry, you appear confrontational and possibly litigious. Refusing to answer would seem downright odd.
To minimize the problem, keep your response brief and positive. Don’t be seduced into a mutual gripe session about the challenges of work-life balance. Make it clear that parenthood has not hampered your success at work. As soon as possible, change the subject to a more businesslike topic.
Technically speaking, the question is inappropriate, but not actually illegal. Legally, the issue is whether this information is used in the hiring decision. But since you can’t use information that you don’t have, smart interviewers avoid asking such incriminating questions.