In this paper, we propose a design method based on the Unified Modeling Language (UML) to formalize information systems dedicated to trace decision making reasoning in serious games. The purpose is to propose a uniform way to represent several serious games. The main goal of our approach is to facilitate the conceptual modelling of the traceability of the stages of the game, the information provided to players and the choices made by them. We also describe a serious game application developed for decision making related to avalanche risk and apply our UML-based method on this use case.