My answer, originally published on Quora.
Don't hide that you are married. And if you are a guy, don't hide that you have kids.
But if you are a mother, the answer is more complicated. Unfortunately, there is a strong bias against female founders (and executives) with kids. In this situation, I would not hide the fact that you have kids but would tend not to bring it up without prompting. If it does come up in conversation, make sure you also mention how lucky you are to have amazing childcare (husband, grandparents, nanny, whatever).