That made me smile. Ancient mythology is fiction.
There are numerous kids’ versions, including d’Aulaire (link: https://www.amazon.com/DAulaires-Greek-Myths-Ingri-dAulaire/dp/0440406943 ), that are pretty light retellings; but bear in mind that many of the tales are not actually G-rated, so a children’s version may be very inauthentic. Reading about the Norse myths as a child, for instance, I thought Loki was a character of fun and laughter. Not so.
Edith Hamilton’s Mythology (link: https://en.wikipedia.org/wiki/Mythology_(book) ) is also a standard. Neil Gaiman incorporated mythology into his American Gods, but I think that book is better read when you know the stories than when you’re first seeing them. I learned mine from Bulfinch’s The Age of Fable, which I read and reread as a kid. Link: https://en.wikipedia.org/wiki/Bulfinch's_Mythology
Also Joseph Campbell’s works will lead you into deep areas of mythology—but I suppose one might consider his discussions and interpretations “nonfiction” even though the subject matter is fictional. His TV miniseries The Power of Myth, with Bill Moyers, is exceptionally enlightening.
The beliefs that have shaped cultures and history are fascinating to me. I have read much on the subject and yet have barely touched the surface. The myths of the Eastern religions are another rich and beautiful trove of human understanding fashioned into stories. The fact that they frequently bypass our logical processes is part of their power to teach and reveal.