I am not a lawyer, but I have done quite a bit of interviewing of people in my time. I was instructed by more than just one company to never ask those types of personal questions, for they could create legal problems.
That said, proving it was because you have children as to why they did not hire you, or for your age, or whatever reason, is difficult.
My personal advise it to just do your best to answer the intent of the question as best you can determine it.
They are likely concerned with your ability to work late on short notice, or if you will be available on a weekend, etc etc, or whatever their intent is.
Simply assure them of their concern, and maybe later on, you could answer the question.
again, in my personal opinion, answer the intent of the question, and I might even look at that rather oddly if they asked questions such as my age or marital status. Oh, I'd probably tell them, but I'd look at them for a second, and make them realize they shouldn't really be asking me that question as it has nothing to do with the job.
just my opinions, and I've both interviewed and been interviewed quite a bit, but this is not legal advice at all.