Participatory Design (PD) has become a standard methodology in HCI, however, the evaluation of the outcomes of participatory processes is often exclusively driven by researcher defined measures of success. Through our work with autistic children, who have radically different life worlds from our own, it became evident that their criteria for the success of a project are most likely also very different. In order to address the limitations of researcher defined and led evaluations in this context, we developed an approach for participatory evaluation called PEACE (Participatory Evaluation with Autistic ChildrEn). Using this approach, we were able to include autistic children in dedicated evaluation phases through the co-definition of goals and methods, joint processes of data gathering and the co-interpretation of results. We discuss three case studies in which we successfully applied our approach and conclude with a reflection on the novel insights created through participatory evaluation and researchers' roles in such a process.