I thought it was illegal to even ask about children ? At least that's what they told me in my interviewing training.
@MrsT : They cannot outright ask, it is illegal, but they can ask leading questions and let the person ramble on. Most people get very uncomfortable during periods of silence. It is an interview technique to use the awkward silence to try to get the interviewee to start spewing information they would not ordinarily reveal, that includes information about children, marriage, etc. You'll see what I mean during your next interview.
A common ploy is for them to discuss their medical insurance benefits, such as how they offer great rates for family plans. They'll give more details until they can figure out if you have a spouse or children from your responses.
Another easy question, what do you like to do in your free time, what are your hobbies? Then you start answering how you love to travel and go snorkeling with your new husband, and you go four times a year to experience as many locations before you have kids. Or, your kids really enjoy going to the zoo and learning about animals. Both answers are going to send your resume into the trash can.