The availability of health-promoting resources for young children diagnosed with cancer who are transitioning from intensive care to everyday life is limited. In the context of designing digital peer support services for children who are considered vulnerable due to clinical and age-related aspects, there are several challenges that put critical requirements on a user-centered design process. This paper reports on a new method for co-constructing child-personas that are tailored for developing health-promoting services where empirical data is restricted due to practical and ethical reasons. In particular, we are proposing to focus children design workshop sessions on salutogenesis, and complement this with a pathogenic perspective by interviewing healthcare professionals and parents. We also introduce the use of proxy personas, and redemption scenarios in the form of comicboards, both collaboratively constructed by children and designers through storytelling. By applying four progressive steps of data collection and analysis we arrive at authentic child-personas that can be used to design and develop health-promoting services for children in vulnerable life stages.