This thesis productively entangles two literary bodies: posthumanist and new materialist theory, and children’s literature and media. Specifically, this thesis applies Donna Haraway’s diffractive (rather than reflective) approach to understanding theoretical concepts and Karen Barad’s agential realist onto-epistemological framework to the figure of the posthumanist child, which I delineate as a figure of multiple materialities, ontological instability, and paradox that appears across children’s literature and media. As the figure of the child has been theorized in multiple ways across children’s literature theory, so do I seek to theorize the figure of the posthumanist child via an application of posthumanist and new materialist concepts to an analysis of selected works children’s literature and media. This set of texts is diverse across publishing era, medium, and targeted age group, but they all feature child protagonists who are paradoxically both human and posthuman in their material formation, and their ontological determination. This thesis argues that through the posthuman-ness of these children, works of children’s literature and media have the capacity not only to reframe our thinking around theories of the figure of the child, e.g. aetonormativity, but also to reformulate the largely childfree theories put forward by posthumanist and new materialist scholars. This thesis ultimately argues that adult theories—posthumanism, new materialism, and aetonormativity—have as much to gain from posthumanist children and their literature and media as the latter has to gain from the former. This mutually beneficial expansion of both schools of thought, via the figure of the posthumanist child, is a vital next step in our cultural approach to the troubles of our current Anthropocenic era.