I used to be a manager. From that perspective, I think the obvious is the way to go. If possible, you make a face to face appointment, but if not, pick up the phone and call. Tell your boss how much you have appreciated everything he has done for you, and that you have truly enjoyed your time there. And that you are sad to leave but you feel it is the right thing to do for your family right now. And that you would be very open to working there in the future if that worked out for both of you.
That's all you can do, really. The rest is up to your boss plus circumstance. Obviously some bosses would be really supportive and there would be no bridge burned whatsoever. Others will think "gee, another woman dropping out for her rugrats." (sigh). If I were your boss, of course I would be sad to lose you but would happily give you a call if something came up later (assuming I liked your work, of course). But I might not be able to call, even if I wanted to, because the economy sucks. So there are my thoughts.
Best wishes on your decision.