Despite a growing interest in player-centred methods for serious games, little is known on how to achieve this goal in practice when prospective users are children. Foundational questions remain unanswered, such as to which design dimensions children should contribute, and how and when they should be engaged. This paper presents the methods and results of two studies that inspired Skies of Manawak, a game for developmental dyslexia remediation. The first study engaged 60 children (age 8–13) in 15 ideation workshops to highlight the story and aesthetics of the game. The second study engaged 258 children (age 8–11) in the comparative evaluation of the game demo with a commercial cognitive training system. The results proved the importance and complexity of the early involvement of children in design. Children strongly appreciated the demo, particularly the story their peers contributed to shaping. However, this story deviated from their desires in several critical dimensions. It had to reconcile gender stereotypes and the violence embedded in their narratives with the game’s purpose. An apparent conflict between designers and children’s values emerged, supporting the idea that children’s engagement in serious game design requires effective mediation to avoid compromising the purposes they intend to achieve.