Hi and welcome, Handmaiden!
I can relate to the issue with very young children. Kids are a handful, no doubt about that. Having a poly dynamic INTO which children are born can be absolutely amazing, but establishing a stable poly dynamic, starting from dating, with small children is a challenge, both emotionally and time-management-wise.
One thing that comes to my mind is that please don't go into having a child with this man if you feel that the poly issue is still out in the open or unresolved in an unsatisfactory way. He might wish that with child-rearing and nesting, your thoughts on opening up the relationship might go away, and they could for a while. They might as easily resurface though, and both of you need to ready for that if you are proceeding with the baby-making plan.
Me: bi female in my twenties