evidently there's something called the dost test in which they look at a list of six items and determine if an image is porn by comparing it to those items. i'm not too interested in spending my morning reading about federal laws, so just using the test as a guideline:

images as you're describing seem to only fit one of the six criteria, "whether the visual depiction is intended or designed to elicit a sexual response in the viewer." this might not even apply because i'm guessing that most of these are just stolen images and that these people aren't specifically taking them with the intent of posting them on whatever forum. er, maybe hoping that these people don't have kids rather than guessing...

if some are taken by the individuals and the children are posed specifically; it could conceivably be called child porn in the eyes of the dost test/law because a child was put into a situation that was meant to be sexual ("child is depicted in an unnatural pose or inappropriate attire", "setting of the visual depiction is sexually suggestive", whether the visual depiction suggests sexual coyness or a willingness to engage in sexual activity") and endangered.

moreover, they're likely just covering their asses by excluding any nudity because that's a red flag not only in the case of the dost test but, like... most site rules and the law itself (in some cases).