The complexity and sophistication of computer games is growing every day, not only in terms of graphical resources and advanced interaction mechanisms but also with regards to their narratives and their rules. In this paper we propose to tackle the complexity related to the rules and narratives of a game design by describing it through combinations of more simple game designs. This approach not only facilitates the description of game designs that are easy to manage and understand but also enables the reuse of game design components, and also allows the designer to respond to established trends in computer games such as the use of "missions" and "mini-games". The proposed approach has been applied to the design and implementation of an educational game (EG) called "Safety Village" of the minigame genre, which aims to help raise children's awareness of emergencies and domestic risks.