In the last years, the growth of serious games has been continuous. This kind of games have been successfully exploited in different areas, so they have been designed with extremely varied features according to the particular necessities of their ‘serious’ purpose. One of the most prolific application areas is the education (educational serious games). Besides, one of the genres that probably better suits to this type of games is the adventure, given the relevance of the narrative in this genre. However, in spite of the powerful tool that this kind of video games has proved to be in the classroom, a few methodological efforts have been conducted in order to involve pedagogues or educators in the design loop of these games or, one step further, include students in the co-design process to promote learning through design as a context. With this main objective, this paper presents a set of metamodels that could facilitate the conceptual design of this type of games. To this end, a complete graphical notation based on the UML standard (with adaptations) is defined for representing the components of this type of games; having in mind to improve the collaboration between the team of pedagogues/students and the technical team during the design process. Finally, the design diagrams defined during the production of a specific serious game, titled Uranus, are illustrated, in order to show the feasibility of the proposal. In addition, a validation experience was conducted with pedagogues and computer engineering students in order to test the value of the proposed graphic notation to design educational games.