The concept of living in a single nuclear family of two parents and their children in a household is mostly a recent and affluent one. It's often not like that in many places, cultures, and times. People live with other generations of the same family, with siblings and their families, with servants, with lodgers, with old friends even. It's not unusual for adults other than the birth parents to be closely involved in a child's upbringing. I haven't researched it, so have no evidence, but I'm willing to bet that some of those households would have involved polyamorous relationships, even if no one else was really aware, because they kept quiet to avoid social censure. But if kids see respectful, loving relationships being modelled, that's better for them than see dysfunctional, abusive ones, regardless of how many people are included.
Polyamorous relationships can come in a variety of configurations. They may not be open relationships at all, but could be closed, just with more than two people. There's a huge range of different ways they can work. They wouldn't work at all for many people. Some might share partners, others could have separate external partners, or they coukd have casual partners. Or a whole mix. It woukd take a lot of emotiinal work, and some people don't do well at that even when there's only one other person. A great many traditional relationships fail; if people going into less traditionsl relationships go into it with open minds, working on it to discuss what boundaries will work for them, how to manage changes over time and any other problems, there's no reason it couldn't last as long as an average twosome. But many people don't put that sort of work into any relationship and go forward on assumptions that might not be shared.