You do realize that lots of people have friends that perform parenting roles with their kids, right? E.g. Couples who are friends that each babysit for each other.
The event of the friendship ending has the same psychological effect on the children as having a poly partner leave the relationship.

A lot of these imagined “harms” and weird situations are regular events that people already experience. It’s the defamiliarization of seeing it in a setting that you perceive as weird that makes it seem harmful and/or weird.