you make the leap from 'sexualised children' to 'child porn' pretty quick... a teen taking a photo of herself in the mirror looking slutty is sexualised and is all over facebook...
Reddit has and will always report child porn and content that they believe is illegal once the users have redflagged it with an admin...
this new policy is about the grey area in the middle, where it isn't porn, but the person is posing in a sexually suggestive manner
people submitting images/browsing /r/preteen_girls creep me out as much as the next guy... but it's not, strictly speaking, pornographic (and it certainly isn't child exploitation if people are simple reposting images that these minors were foolish enough to upload to the internet in the first place)