Educational escape rooms are being used in all school sectors and in many different subjects across the globe. In most documented cases, escape rooms are designed by teachers and played by students. This paper presents the case study of a primary school class that designed and developed an escape room for both adult and young players in connection with a regional event connected to a historical celebration. The project spanned over the whole school year and included a learning phase, the production of support materials (as comics) and eventually the design and development of the escape room, which was finalized by a local association. After its installation in the ancient local prison, the escape room enjoyed the visit of over 600 players, including tourists, families, groups of friends, and school classes, representing a successful example of open school game design project. The paper presents the key phases of the project: requirement analysis, escape room design (including narrative, puzzles, game-flow and props), escape room implementation, and assessment, with a focus on how to manage complex game design with young children.