It's been over 6 months since I've posted, but this is so similar to what I'm trying to figure out now. My girlfriend and I are serious, but my 13 year old (I've got girls 16,13, and 9 year old twins) is throwing a fit becasue she's just starting to realize that girlfriend could in some years theoretically be step-mom. It was OK when my relationship was out of the house, but now that I even want to have the kids get to know her, the walls are going up.
This phase is just starting, but I appreciate reading how others are handling it.