The Conan stories were certainly written for adults. More of niche market than today -- "geek chic" wasn't exactly a thing, but it was not for children. Edgar Rice Burroughs, I'm mostly familiar with the Mars books, which I don't see as being for children, either, particularly with the "oh, yeah, and everybody's naked on Mars" stuff.

The Hobbit has some claim to being children's fare, but it's followed by The Lord of the Rings, which is less so. Lord of the Rings had a bit of a counterculture/hippie following in the 60s.