Specifically a rule banning it so we don’t get defederated, coupled with a clarification about what it means to fall under this category.
I’m thinking of situations like characters that look underage but canonically aren’t (i.e. the 1000 year old loli trope) and characters that are drawn indistinguishably from adults but are canonically underage.
I believe this is a very hard to determine thing, probably the most effective way could be the age limit but as the Fediverse consists of a unregulated thing at least for a specific legal jurisdiction it could be complicated too, it could be determined by the rule of not abusing someone but if you do that in consequence drawings, AI generated CP and some other IRL content could be allowed, so in my opinion pedos should make their own unfedered pedo related instance.