Having worked with transposition of domain knowledge in digital and card games, we propose a novel approach for enabling groups of primary school pupils to express their shared understanding of a topic; the group can represent their knowledge by creating a trading card game (with custom cards and rules) instead of using diagrammatical formalisms. A kit and a special design method have been devised to simplify the creation of card games, bringing the task in within the capabilities of pupils. The process of designing card games represents in itself a form of group reflection in action. The resulting card games serve as boundary objects among learners and instructors (or other participating adults, in informal contexts). The games reify the group knowledge, making it tangible and playable: each of these games can be seen as a simulation or a presentation of knowledge, for the benefit of new learners or for assessment. Studies are currently being conducted to investigate the full potential our new knowledge, and how to improve our kit further.