The United Nations Convention on the Rights of the Child was the first international treaty to emphasize the importance of listening to children's views and including them in decision-making processes. In that line, the primary aim of this study was to show how the quality of preschool outdoor environments can be improved through a participatory intervention involving children, parents and teachers. A total of 1001 children aged between 3 and 6 years who were attending seven different preschools in the Basque Country (Spain) took part, along with 54 parents and 94 teachers. The data collection methodology included participant observation, images, photographs and drawings, as well as conversations and interviews with children, parents and teachers. The quality of the school outdoor environment was evaluated using the Preschool Outdoor Environment Measurement Scale. The results showed that the intervention led to improvements in the outdoor environment of schools, thus underlining the value of using participatory methods in which children are involved. In our view, the participatory approach not only respects the UN Convention on the Rights of the Child and helps to promote democracy in schools but also draws upon the creative potential of children to transform the world around them.