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This article focuses on the medium- and long-term effects of co-designing a serious game involving children with hemophilia, their parents (fathers and mothers), and their teachers. Our study aimed to permanently modify the mental representations of parents and teachers by involving them, along with children, in the co-design of a digital environment (here, a serious game) intended to explain how and why young people with hemophilia should practice sports at school. Our study is based on focus groups and a methodology based on a pre-test and three post-tests. Our results show that while participation in focus group sessions during which a serious game is collectively designed changes participants' opinions about the importance of physical activity for children with hemophilia, the medium-and long-term effect is much more nuanced, because several parents (fathers, mothers) and several teachers return to their initial opinion, namely that “physical and sports activity is not important for children with hemophilia.”