A.S.
I agree with previous posts. The key in the situation is drawing boundaries and sticking to them. I always intended on returning to work part-time after I had my first child (and will again after I have my second). I was very worried that it would be too much, esp since there is an expectation in my small company that you will always be available for whatever work they need you to do whenever they need it: weekends, late nights, what have you (it's a 24-hr operation). I found, however, that they were actually very understanding of my part-time situation and there have been very rare occasions when I've been asked to work late, or a little from home on days I don't work, but it's not expected at all. I've actually volunteered to come in, if needed, for some weekend project work that I used to have to come in for, and they've said not to worry about it. It's has turned out to be a very positive situation for me and, similar to your situation, I am only doing the part of my job for which I'm soley qualified and necessary to the company and the part that I most enjoyed. The other headaches and nuisances have been passed along. I feel like it's a great situation and you'll obviously be in a good position if they are seeking you out to come back. They find you valuable and in that position you can and should most definitely dictate exactly how much you're willing to do. The hardest part for me was actually adjusting to letting go and passing along the stuff I was so used to doing, but did not like and could be done by someone else.
Good luck with your decision!