I wanted to thank you all for the thoughtful discussion here. My primary and I spend a lot of time talking about what, when, and how to talk to our spawn about other people who are part of our lives. Right now, it's important to me that we keep his expectations in check; he likes to play with my girlfriend, who's here with us several times a week, but her role is definitely that of a family friend. My relationship with her is new(ish), so while I think it's good for our son to interact with her, I also want to be clear with him that, while we love girlfriend (I have got to come up with some nicknames for everyone!) my primary and I are his moms who will always love him and never go away. We try to talk to him about how friends can come into our lives and sometimes go out, and that's ok. We make sure he knows that while some relationships can change or even end, his relationship with us will never stop. I'm somewhat affectionate with girlfriend while my son is around, but it's the kind of affection I could also share with my best friend -- hugs, head on a shoulder kind of things, but not kissing.
Several people talked way back at the beginning of this thread about not wanting to give their children the challenge of being different. I certainly understand that, but since my son has two moms, we're already different. I don't get fussed about it. I do get fussed about secrecy. I don't want to burden him with my secrets. I think that's part of what's motivating me to be more open about polyamory lately. The secrecy thing isn't an issue right now, but I can imagine it becoming one down the road if we aren't completely out.