The problem with posting pictures of kids in closed groups is that pervs will just join those groups because they have what they're looking for. You're basically making it easier for them.
It's not that parents are afraid of their kids being part of a training set, though that is a bad thing in and of itself. It's more about all of these AI undressing app ads that are showing up on every social media site, showing just how much of a wild-west situation things currently are, and that this brand of sexual exploitation is in-demand.
Predators are already automating the process so that certain Instagram models get the AI undressing treatment as soon as they upload an exploitable pic. Pretty trivial to do at scale with Instaloader, GroundingDINO, SAM, and SD. Those pics are hosted outside of Instagram where victims have no power to undo the damage. Kids will get sexually exploited in this process, incidentally or intentionally.