I've been getting a lot of these showing up, and would like to add them to my blocklist, but I'm not trying to block all young images, as some are just cute stuff. What tag do I add to the list?

young -rating:s, it should've been on your default blacklist. this will hide any explicit/questionable image containing young characters while still allowing the safe rated posts.

Also try chibi -rating:s since there's a lot of overlap and posts missing the young tag. There's also a few pokemon species that are similar- riolu and pichu are probably the most frequent ones.