Recent immersive theatre practices being developed by UK theatre companies invite children to believe that a book story has come to life and is really happening to them within their school environment. The thrill of being able act within a story motivates children’s embodied agency to physically explore an immersive set installation and acts of creative writing performed during the story experience, which allow children to co-create the narrative alongside the adult practitioners. These acts are fictionalised into the narrative frame of the children as heroic story writers, offering new perspectives on how child co-creation of adult narratives can be enabled intra-textually, within the bounds of the adult-initiated story, rather than extra-textually after it has finished, as is the case with fan fiction. However, it raises questions about the nature and limits of this co-creative agency the experiences purport to elicit from their child participants, especially in terms of their critical agency to manipulate fictional devices presented to them through such a hyper-real aesthetic. Drawing on theories of children’s literature and participatory performance, this article argues that it is the way these immersive productions uncannily overlay real and fictional temporalities that enables the child participants’ critical agency within them. Approaches for further participatory, practice-based research in this understudied corner of children’s reading universes are suggested for harnessing the potential of immersive story worlds to generate research data from within children’s first aesthetic engagement with stories.