"real children should be the priority"
While this point sounds good on paper, I don't think "fictional / non-existent children" should be "protected" at all, let alone be merely deprioritized. Perhaps, that is the intent here (and it would jive with other posts), however, this language is ambiguous.
Freedom of expression is a fundamental human right.
There is no valid argument to the contrary. Even the debunked* "what about criminals?" would be highly disproportionate, even if it were not debunked (and predicated on a form of "guilt by association" which violates due process).
Nothing stops actual criminals from being punished, particularly the worst ones. Most people are not going to be criminals anyway, especially not worse forms of criminal... Indeed, one psychologist noted that crimes in one of these populations were "so rare".
Also, undermining fundamental rights means undermining them for everyone in the long run... There is no such thing as a benign exception here... This has been shown in multiple areas.