Even in light of US vs Knox, which I had not know about, the material on r/jailbait was not child pornography. The courts opinion list some criteria for something to be considered child pornography. The material on r/jailbait does not fit these criteria.

1) whether the focal point of the visual depiction is on the child's genitalia or pubic area;

2) whether the setting of the visual depiction is sexually suggestive, i.e., in a place or pose generally associated with sexual activity;

3) whether the child is depicted in an unnatural pose, or in inappropriate attire, considering the age of the child;

4) whether the child is fully or partially clothed, or nude;

5) whether the visual depiction suggests sexual coyness or a willingness to engage in sexual activity;

6) whether the visual depiction is intended or designed to elicit a sexual response in the viewer.

Opinion: http://www.mit.edu/activities/safe/cases/knox/94a0734p.htm