The thing I don't like about how this thread has gone is that, for the most part, when a monogamous couple wants to have a kid, the only question that gets asked is whether they can afford it or not. But mostly they will be cheered on and congratulated without any wagging of fingers or warnings about the stability of the relationship - and many mono couples want kids just because they have been taught that that is one of the goals of getting married or settling down. Few ever ponder their reasons for wanting to bring another human being into the world or choose the path of becoming a parent with much forethought beyond meeting family expectations or what color to paint the baby's room.
Now, here you are in a solid, loving poly situation and posting at a polyamory-focused forum, and gettting a lot of negativity and forewarnings. I am childfree by choice myself, but am very surprised at the responses you've gotten thus far. I think that if you have been able to make a complex poly arrangement work well, you will be able to make raising another child within that arrangement work well, too. It is obvious to me that you are thoughtful, concerned, and not just blindly jumping in. We sometimes get poly newbies who post here all enthralled with their new lifestyle who say, "I wanna have my bf's baby!" And you can tell that they're not really using much common sense. But I don't sense that about you. You seem to have your head screwed on right, and I believe you will be able to negotiate any difficulties well.
I hope that you get more feedback from people who are successfully parenting in a poly household.