... I am lost as to why that is a blatantly illegal question. I am interviewing for a au-pair right now and it's a very relevant question, as I sure it would be for any job that it's critical to have people present for (ie if I know you have kids, then I can make sure if you have a family emergency, I can cover it) etc etc.
However.. "are you planning to have kids ?" ... now that is a different matter.
I was just browsing job ads on Craigslist... which I do occasionally just out of curiosity of what local tech companies are hiring... and was surprised to see more than one asking that you send your age with your resume. That's just asking for a lawsuit when anyone over 40 doesn't get hired.
However.. "are you planning to have kids ?" ... now that is a different matter.