What do we know about immersive media--virtual, augmented, mixed, and cross realities (VR, AR, MR, and XR)--and young children? So far, designers, developers, and media producers have been focusing on creating hardware, software, and content for and conducting studies with adolescents and adults--but children find these technologies incredibly appealing. How can we prepare for a future in which immersive media are readily available and a more common part of daily life? On November 6-7, 2018, the Joan Ganz Cooney Center at Sesame Workshop, the School for the Future of Innovation in Society at Arizona State University, and Dubit hosted the inaugural Future of Childhood Salon on Immersive Media and Child Development. Over the course of a day and a half, a cross-sectoral group of approximately 60 child development and media researchers, learning scientists, child health experts, hardware and content developers, educators, journalists, and funders examined these emerging media through various activities, organized around how to ensure VR, AR, MR, and XR are safe, engaging, and beneficial to children's socio-emotional, cognitive, and physical development and learning. This report synthesizes the proceedings of this event, in which participants discussed an agenda for needed research and considerations for responsible development of immersive experiences for children.