There is a growing focus on computational thinking (CT) in terms of supporting children's understanding of everyday technologies. But unlike other technologies, Augmented Reality (AR) has received limited attention. In this paper, we present ExposAR - a collaborative cross-device AR system enabling children to create, use, and reflect on AR technologies through the co-creation of a simple AR application. With ExposAR, we explore three core design principles: 1) reify computational concepts, 2) support collaborative cross-device authoring, and 3) incorporate the user's own world. These principles were developed through a co-design process with teachers and evaluated with 46 primary school students. We found that the collaborative authoring process with ExposAR supported students in understanding AR concepts and challenged their perspectives on AR. With these results, we bring AR to the CT agenda and contribute novel design principles for exposing the underlying mechanisms and implications of AR.