The novel is about colonists on Venus. The main characters are two young boys. The boys make friends with a comical, monkey-like primitive native, and also meet a crazy human hermit character. The characters are captured by a hostile native tribe, and offered to the tribe's giant snake monster god. As I recall, the hero kills the monster with a grenade. I have tried several online searches using these plot points, with no luck. I read this novel when I was in the fifth grade, as part of an advanced reading program, but the book's style seems earlier, from the 1940s or 50s perhaps. Thank you!