Having kids will definitely change the relationship between the biological parents. It creates a bond deeper than ever as they now (hopefully) have a shared goal of raising another human being. Any two people having sex should have a plan in place for pregnancy.
If a woman has multiple, polyamorous partners then perhaps the father is unknown. The main thing a child needs is a role model not a "father". I grew up without my father but I had other families and men in my life to show me exemplary behavior, caring for others, honesty, confidence etc. If one of the men is a "primary" partner then perhaps he takes on the father role. There is no reason that other loving and caring men in the child's life will do harm. Watching positive interaction, caring, and cooperation between their "father" and the other man or men can only be a good thing.
In the opposite case where a man has multiple partners the same applies (except of course the mother is definitely known). Other loving women assisting cant be a bad thing.
In the case where 2 men and 2 women are a group (or some other mixed arrangement) all of them may take on some of the parental roles. Since our society is so structured as to require a "mother" and "father" (for school and other friends) it is likely 2 of them will be the "official" parents on paper. I read a story about such a group (2m 2f) but the shape of their relationship was more of a line than a box with a cross in the middle. So the two in the middle got married but would have liked to include an other woman in the marriage as she was equally a mother (just not biologically).
Personally I think the more a child is interacting with humans and not sitting in front of the television the better. To raise a child really well takes an enormous, extended, loving effort and this will only be easier with more than 2 people.
The polyamorous idea of "compersion" or happiness at the happiness of others leads me to think that anyone properly practicing polyamory in an open and honest way would only be glad to be a part of raising the child of one their partners.
This isn't just a fantasy dream, people actually do this. The idea of the "nuclear" family is very new, in ancient times children were breastfed by whomever could do it, raised by a small close-knit community brought ever closer by the task of raising an awesome human.