In this study, we present Coco's Videos, a video-viewing platform for preschoolers designed to support them in learning to self-manage their media consumption. We report results from a three-week experimental deployment in 24 homes in which preschoolers used three different versions of the platform: one that is neutral to the limits they set, one that enforces the limits they set, and one that attempts to erode the limits they set by automatically playing additional content after the planned content is finished ("post-play"). We found that post-play significantly reduced children's autonomy and likelihood of self-regulation, extended video-viewing time, and led to increases in parent intervention. We found that the lock-out mechanism did not reduce video-viewing time or the likelihood of parent intervention. Together, our results suggest that avoiding platforms that work to undermine the user's intentions is more likely to help children self-regulate their media use than rigid parental controls.