If there is any "need to know" then tell them. Things like "I'm sorry, I can't stay late as I need to pick up my son at daycare by 6." I'd let it come up naturally. I think the concern comes from people abusing the good will of employers by pulling the kid card, but if you prove you are a good worker and all that, then don't hide the fact that you have kids. A lot of people have kids and work FT. I do agree that if you are nursing and pumping, they should know simply to allow you the time by law to pump. And it might help if your boss knew you had an infant regarding all those first year dr's appointments. Not as an excuse, but as a fact.
However, I have also learned to keep most of my private life private. They don't need to know when he kept you up, when he learned to potty train, etc.