We investigate the concept of care in adult-child joint play through two cases that illustrate ways in which the development of care relations among researchers, pedagogues, and children—and the imaginary characters they create through their joint play—shape and sustain early childhood education and care research and practice. We focus on the ways that early childhood education and care pedagogues’ approaches to care provide insights into practices of social sustainability, specifically social inclusion. The cases we present are drawn from recent studies of early childhood play. The studies belong to a corpus of international research projects that are researcher-teacher collaborations. These studies explore a unique form of adult-child joint imaginary play known as playworlds. Playworlds are based on cultural historical theories of development and art, Gunilla Lindqvist’s studies of playworlds, and local theory and practice of early childhood education and care. Our analyses of playworlds are based, in part, on Winnicott’s concept of transitional objects. The two cases are drawn from ECEC playworlds in Finland and the US. Each exemplifies how playworlds, as forms of participatory design research, make social sustainability possible. Furthermore, these cases highlight how, by working with the boundaries between and moving between real and imagined, the participants are able to develop new ways of being that are radically inclusive. We argue that they do so by facilitating and maintaining the development of care relations among researchers, teachers, children, and, importantly, imaginary characters, in ways that create what we call transitional subjects. We conclude that social sustainability, like care, should be conceived of as an ecology of caring practices.