Ignoring the legality of it for a minute. The title "Jailbait" implies sexualization (My spell checker insists this is not a word, but it's meaning should be obvious to infer.) of the images within. Even if the images were all "legal" by the standards of United States law, (I'm assuming Reddit is hosted in the US.) they're still being used to fuel sexual fantasies related to the images. As I understand it the reason that child pornography is illegal is because it's distribution and consumption encourages the sexual abuse of children worldwide. If the end result of the Reddit "Jailbait" subthread is sexual fantasies of children, then the damage done is the same. For example, if the FBI seizes your computer and finds a file labelled "Porn" on your hard drive full of images like children tricycling around a neighborhood, your almost certainly going to be asked about it in the police interview(s) even if it's not necessarily prosecutable.
Everything thats illegal isn't necessarily immoral; and everything thats legal isn't exactly moral either. The sexualization of children is immoral, legal or not.
True. However, the years between childhood and adolescent are blurry. This is the reason you have the term "Jailbait" in the first place. Some people are "late bloomers" and look like children well into their teens. Others end up looking like they've reached the age of consent, but haven't; the "Jailbait". I think that in general sexualizing the under 18 crowd should be avoided. (Even if the age of consent is lower than that.) Just because of how blurry these lines are.
Everything thats illegal isn't necessarily immoral; and everything thats legal isn't exactly moral either. The sexualization of children is immoral, legal or not.