Should be photos/manips/images (including, technically, drawings) of actual people, *not* depictions of entirely fictional characters; that's what US law says.

For the purposes of this chapter, the term—
(1) “minor” means any person under the age of eighteen years;

Fictional characters are not "persons;" they have no legal rights. Among other things, authors are allowed to torture & kill them. The laws against child pornography in the US are part of child abuse laws; no real child = no abuse.

(IANAL. IAN an OTW representative. Just someone who's been researching the legal aspects since Strikethrough.)