oOoOoOoOoOo... I'd love to share your excitement, but I've seen too many horror stories start out this way.
Fooling around is very much different from having a loving, open & honest romantic relationship. Make sure everyone is emotionally prepared and up to the difficult task at hand. Make sure this couple REALLY REALLY WANTS to be polyamorous, talk to the wife alone and get a really good sense of what she truly wants, before getting yourself stuck in an undesirable situation.
This situation will require everyone's communication skills to be sharp and ready. Everyone will need to fully disclose how they're feeling at every step along the way, without fear of being judged.
The whole "roommate-slash-lover" thing is sorta playing with fire. It has gotten many people into situations where the romantic part didn't work out as they'd expected, and then they're scrambling for a place to live, or else stuck with "nowhere to go" and trying to pick up the pieces at the same time as they're still falling apart.
The whole "they are deciding on an open marriage" thing implies that they are new to this. My gut reaction is that before moving in with them, it would be better for them to get some experience dealing with polyamory, and for you all to get some experience dealing with romance without the additional burden of living together.
I'm concerned about the child involved. He may grow attached to you, and if it turns out that polyamory doesn't work for this couple, things can get ugly and I would hate to see a young child caught in the middle of all that. Children have a strong sense of tension, even when you think you're keeping it from them.
Gralson: my husband
Auto: my girlfriend