Social support is essential for a person's capacity to live with a disease. For children, who do not have the same ability as adults to understand and deal with their disease, contact with other children with similar experiences can help them understand their own situation. In this paper we present an ongoing research project with a participatory approach. The challenge is to design online peer support (OPS) for children who have been diagnosed with cancer. The aim of this paper is to identify and shed light on ethical implications and related research considerations when designing a research approach for this sensitive context. Our literature review indicates that there is an apparent lack of research on OPS for supporting children with cancer. It also shows that there is knowledge on considerations to make when designing with children. However, the children in this category are suffering from physical and psychological consequences of their illness. Finally we discuss relevant ethical considerations, of which we highlight sensitivity and respect as most important and comprehensive.