Read first, or watch first?
This is the rare case where movie first is genuinely a fine plan. Most kids arrive already knowing the 1939 film, and the book treats that as a head start: the opening feels familiar, and then the story keeps going into chapters no one has spoiled. One thing worth saying before they start: the movie makes Oz a dream, and the book never does. If your kid found the book through Wicked instead, it works just as well in that direction; this is the original map of the whole country.