Karl Rahner’s ‘Ideas for a Theology of Childhood’ has become increasingly influential in childhood studies. In this article I critically assess Rahner’s ideas in this seminal work to highlight the answer Rahner proposes to the question of the meaning and task of childhood. This is brought into brief conversation with writings of Jürgen Moltmann and Jerome Berryman. Rahner’s ideas of childhood can be critiqued for a lack of careful engagement with the details of Scripture and concrete life settings, and for underestimating the effects of childhood trauma. One can also ask whether transcendental philosophy provides an accessible language, particularly in an interdisciplinary context. Nevertheless, ‘mystery of the child’ language is now near-ubiquitous in contemporary Christian authors on childhood. Rahner’s insights into childhood can function like the reality of childhood itself, remaining as a life-giving root in a theology that unfolds as others take it further.