A recent Reddit thread discussed what happens when a pair (of I'm assuming primaries) aren't on the same page with regards to having kids. I don't want to talk about that person's specific problem, I don't have a lot of details and the details I do have make it seem pretty hairy. So let's talk in general terms - from the male perspective as that's the only view I have to share.
Personally, I never wanted to have children. Until last year, I was still capable of producing children. Why?
Getting a vasectomy is psychologically difficult. I had to spend a lot time reading up on it to educate myself on what was going to happen -- and even then I wasn't sure I believed what I read. It was a tough decision, and it was something I had to decide for myself. (There were also cases of the doctors saying "You'll change your mind when you're older." or "What if you meet a woman who wants them?" It took a while to be old enough to class out of one claim and push through the other.)
For that reason, I feel when a couple decides not to have children, it's unfair to pressure anyone into taking surgical steps to make that decision permanent. In polyamory the desire to exert that pressure is likely greater because of a fear that your partner may make a different decision with another person.
I can empathize, having a child is a huge time and resource commitment. If they do that with someone else, it's taking away from your time and resources!
Did you notice the mistake in thinking there? It's easy to feel like you somehow own your partners attention; but you don't. The time and money a person chooses to spend with you is a gift; you're not entitled to it. I would argue strongly that this is an axiom, even in monogamous marriage.
It's also possible that someone could take it as an insult. "They don't want to have kids with me because I'm old!" or "They don't think my genes are good enough for their kids!" or "They think I'd be a bad parent." Ultimately, I think this is jealousy talking. You are who you are, and your relationships are what they are. Wishing they were something different isn't going to change that. Getting angry over relationships you're not involved in will generally diminish the quality of the ones you are in.
So, how do you get on the same page as your partner? To be honest, I haven't given this much thought. I don't want to have kids, and I tend to date people that already have them or are also committed to not having them. But let's say I change my mind tomorrow and the doctors are able to reverse my vasectomy. How would I go about it?
First I'd talk to my partners. If I have a partner I'd like to have a child with, I'd talk to her first. Once she and I are on the same page, I'd begin to discuss it with my other partners, and my metas on my baby mama's side. What concerns do they have? What can I help with? What are outside of my control? Will the new situation bring an end to relationships as they are? If so, can we part amicably? What do I think their involvement will be in my child's life? What do they think their role will be? It will all need to be discussed. New boundaries will be created and enforced. Dynamics inside each relationship will change. That's the nature of the project, and what I'd be signing up for if I decided to undertake it.
It seems deciding to have a kid is a larger decision here than it might be for monogamous people. For that reason, it's important to know your partners' position on the subject as their attitudes shift. It may have an impact on your relationship and you may need to be ready for that. However, you must remember that you never have authority over someone else's time, money, or body.