It just depends on the people involved. Our friends have 2 kids. These two kids are one mommy and two daddies. They all live together. The two daddies aren't romantic with each other, but they are family and are both romantically involved with the mother. One of them fathered the children, but is perfectly happy and okay with the kids calling the other man daddy as well. they differentiate by saying "daddy Steve" and "daddy John".
as long as there isn't sex in front of the children, or loud with the kids in the next room, there is nothing wrong with having poly relationships and children. As long as all people involved are dedicated to the kids, it's beautiful to have them grow up knowing that so many different kinds of love exist in the world.
With that same family, there was a 3rd man involved, and he was irresponsible. He wanted to be daddy as well, but he kept being absent, and so everytime the little boy saw him and he left, the little boy felt abandoned. They finally stopped involving the 3rd man in the child's life as a father figure. In a few years, when this man has gotten his act together better, he can be an uncle of sorts, if he wants, but the kids needs to heal from the emotional abuse of being abandoned several times.