Brain Quest is an active smart phone game designed to promote both physical activity and executive function in 10-11 year old children. This paper describes the user centred design process which involved a team of psychologists, HCI experts, physical activity specialists and thirty four children over a period of 18 months. Results of two preliminary studies are promising, suggesting that BrainQuest is enjoyable, promotes moderate physical activity and has the potential to provide cognitive scaffolding of the key executive function (EF) skill of multitasking. Categories and Subject Descriptors H.5.2 [User Interfaces]: Prototyping, User-centred design General Terms Design, Human Factors Keywords Children, executive function, game, physical activity