I read this book when I was a boy (85-89)but it wasn't a children's book. It had a blue sky and a tree on the cover. An astronaut was reaching up to an Alien in the tree. Written in English. It was hardcover but shorter and smaller than a normal hardcover novel.
The opening chapters deal with the discovery of an Earth-like planet. A team of explorers is sent to investigate and they discover a race of humanoid aliens.
My biggest takeaway from this novel was that the aliens have elongated arms. I also think they may have lived in trees.