Child porn was completely legal through the 60s and much of the 70s.

While it's true there was no FEDERAL child porn law until the late 1970s, many states and localities found ways to make it illegal.

Prior to the 1950s or 1960s, it wasn't much of an issue. I don't know if this was because it was too expensive to make, if nobody had though about marketing to the American audience, if laws on the books at the time sent a "chilling effect" through porn shops, or if society was such that nobody would dare sell such goods. Then came the sexual freedom of the '60s. People marketed KP and many (but not all) states found their laws either ineffective or unconstitutional. Over time, states drafted new laws so that by 1977 KP was legal in only a few places in the USA. In 1978 the first federal KP law effectively banned KP nationwide. There may have been a little quasi-legal action after that but that lasted only until the courts upheld the federal statute.