This project takes as research object the public spaces and artifacts designed for children’s play activities, as well as the ways in which they are occupied and used. Planned or absent, structured, or not, these places and artifacts are analyzed to discuss the right to the city of children living on the urban margins in Brazil and Portugal. In dealing with this object, in all its particularities, and adopting a dialogical-practical perspective, we have mobilised a series of authors who deal with the topics of childhood and the right to play. Moreover, we have brought those who discuss the development of the city from the time when it was a place for exchanges and meetings, and during which use value prevailed, to contemporary times, when exchange value and market interests prevail. Guided by participatory design, we value children’s tacit knowledge and capacity for agency in the diagnosis, planning and co-creation of play spaces and things. Taking as our empirical universe two neighbourhoods located in Greater Lisbon Metropolitan Area (Bairro da Cova da Moura and Bairro da Torre) in Portugal, and two in Brazil (Vila Maresia and Bairro da Liberdade), located on the island of Maranhão, we have developed four social design experiments. These rehearse ways of designing that result in socially useful things, with agency, creativity, desires and needs prevailing and respecting the individualities of participants, pointing to devices and guiding principles for the participation of children in processes of thinking about places and artefacts for play with a view to the co-creation of things to play with, especially in marginal areas of cities, but not only these.