Mobile outdoor games can be seen as fertile ground for conducting novel learning activities that involve children in different tasks including physical motion, problem solving, inquiry and collaboration; all those are activities that support different cognitive and social aspects of learning. Co-design and human centric design practices have been the focus of current research efforts in the field of educational technologies but not as prevalent in mobile games to support learning. In our current research we are exploring which design methods are appropriate for developing innovative ways of learning supported by mobile games. This paper presents all those aspects related to the design and implementation of a mobile game called Skattjakt (Treasure Hunt in Swedish). The outcome of our activities has provided us with valuable results that can help us to bridge the gap between learning in informal and formal settings. Moreover, we believe that involving children in the design process of mobile games may give us new insights regarding the nature of their learning practices while learning with games.