I read this book in six grade in the mid-80s. I think it was a fantasy type story. This society had once been in contact with humans (or maybe just one human). He may have come over the mountains on a hot air balloon.
They got a few things confused about humans. For example, they had a painting of a tree and a diagram of a family tree (genealogy chart). They called the genealogy chart a picture and the painting of the tree a family tree. So everyone in this society had a very carefully tended “family tree” in their yard. Unfortunately, I remember nothing about the actual plot.
Heh, I bet I’m going to be the umpteenth person with this answer, but:
The Gammage Cup, by Carol Kendall
https://beastiaryofbooks.com/2018/07/17/the-gammage-cup-1959/
Oh my God!! I think you’re right!! This has been bugging me for more than a few decades!!!
Definitely it.