This study examines the work of usability specialists in a multiparty research project producing an open source learning application for children, with children. Children as a user group has been acknowledged decades ago and methods for involving them have been devised, but there is a lack of research examining what happens to children’s input in practice, when integrated with actual development. The paper contrasts the empirical findings with the existing research on the usability specialists’ roles and with the knowledge management literature on boundary spanning, which argues that for successful knowledge sharing and arriving at shared understandings there needs to emerge boundary spanners and boundary objects and a new joint field of practice within which the experts involved can collaborate. This paper argues for the boundary spanner position to be acquired by usability specialists. Instances of successful boundary spanning are described and conditions for successful boundary spanning are discussed.