"There are major mainstream erotica sites hosted in the US that allow child erotica in addition to adult."

Major sites, I've heard of.

Mainstream, as in "commercial enterprises linked to a fixed address" who will stay put long enough to be answerable?... That I've never heard of.

Some sites allow anonymous nicks to upload all manner of posts. But we cannot assume that "hasn't been shut down" is the same as legal to publish; particularly if you do so under your real name and seek real profits for your writing.

Phillip Greaves was arrested for less.

Dante