This paper presents the Collaborative Design Thinking (CoDeT) co-design approach, its theoretical framework, and its application in a case study with 49 children aged 9 to 10 in two schools. CoDeT aims to scaffold children's collaboration and design thinking in co-design settings characterised by a high child-to-adult ratio (ca. 1 adult for 15 to 20 children), such as schools, museums and maker spaces. In these settings, children have to work relatively independent from adults who become guides on the side. This can be challenging due to children's limited understanding of the design process and their lack of skills to collaborate productively towards a shared design goal. CoDeT addresses these challenges by integrating principles of Social Interdependence Theory (SIT) and Design Thinking (DT), which together form the theoretical backbone of the approach. CoDeT was first applied in a case study and yielded promising results in terms of children's collaboration and design thinking skills, yet possible improvements were found. The insights of this case study informed the revised version of CoDeT presented at the end of this article, in a what-why-how structure, allowing researchers and practitioners to apply the co-design approach in a wide variety of contexts characterised by high child-to-adult ratios.