Why not require a negative prompt leading with child, kid, in the first 10 words? I'm not good at writing a bot, but it might even be possible to intercept images to check that this exists in the meta data before Lemmy wipes it....maybe. It might exclude a few ups, but it would greatly reduce the moderation load.
I don't think forcing people to use specific negative prompts is a good approach. Don't take that as me saying I think CP is acceptable though. Also people can easily fake metadata on images in the same way they can modify an image. Using a bot to check metadata won't solve anything. Plus you could probably make CP looking stuff even with specific negative prompts.
Downvote and reporting is best in my opinion.
but it might even be possible to intercept images to check that this exists in the meta data before Lemmy wipes it....
This would be impossible without directly modifying Lemmy itself.