... 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.