I don’t think the research actually supports this. The emerging research suggests the opposite — that children with more caregivers in their lives tend to thrive. Whether from same-sex couples or in a polyamorous relationship, when children have attentive and present caregivers, they tend to have more successful life outcomes. As a child from a dysfunctional monogamous (and failed) marriage, I can see how polyamory can be more conducive to parents maintaining long-term relationships and helping children thrive.
See this article for example -
https://amp.theguardian.com/lifeandstyle/2020/feb/01/zero-evidence-worse-for-children-parenting-in-polyamorous-relationship