Today urban areas are growing fast, in this process it remains a challenge to include the opinions of the public. This holds especially true for young people. From the 1960's, the rights of the children started to be recognized through the UN Convention on the Rights of the Child. Article 12 of the convention states that children should have the opportunity to express their views in matters that affect them, like urban planning. Our work started analyzing the literature and providing an overview of participatory methods used to include children in land-use projects. We then reviewed existing mobile apps targeting participation in public life. Based on the findings from the literature a list of high level requirements was created to guide the design and implementation of the mobile app DELTA. The DELTA app support situated participation encouraging users to explore the urban environment, promoting awareness and critical thinking. Several user evaluations are performed during the development cycles, including expert evaluations, usability and field tests of the final prototype. Based on collected data and lessons learned, results are discussed in relation to participation and learning outcomes for children.