Hi Turtle2 -
I was able to locate a number of online resources on the topic of polyamory and ethical non-monogamy which were ostensibly for children (or young adults). But in browsing through these it turned out that there were actually very few such books, while there were a number of books which were deemed useful in opening a child's mind to other-than-conventional families. But some of those other-than-conventional families were simply same sex relationships of a dyadic sort, etc. Most of those books didn't really fit the bill.
Could you do us a favor, after having read through those materials and otherwise searched, and let us know which books actually fit the bill of what you were wanting?
Who knows? Perhaps some writers and publishers are reading here? Perhaps we will inspire some to write and publish more children's books of the kind you are wanting.