The well-being, activities and social integration of young children closely depend on the social and physical characteristics of their immediate environment. In the course of the twentieth century, the representation of the early childhood has evolved in a striking way, and this evolution has entailed important changes in the environments designed for young children. We first show, in Section 1, how a restricted medical vision of childhood promoted a model of day-care centers protecting children's health but providing them with a poor developmental context (1950-1970). A second section presents the results of two empirical studies carried out in French day care centers at a ten-year interval (1985-1995). These studies revealed that friendly interactions among toddlers were reduced by half to the profit of parallel uses of play material. As regards social sustainability, our results raise questions with respect to the design of environments in which the social relationships of the generations to come develop.