While developmental discourses have been heavily critiqued in relation to education systems, less attention has been paid to how these impact the data collection process in classroom research. This article utilises Foucault's concept of regime of truth to highlight the pervasiveness of developmental discourses when conducting research in primary schools. Such a theoretical framing makes explicit how developmental discourses work and are constructed as "truth", which limit the possibilities for alternative perspectives. This article shows how this regime of truth works in practice by reflecting on qualitative research conducted with two age groups in two primary schools in Australia, focusing on the researcher's navigations of these discourses. In particular, this article examines the impact of developmental discourses on conducting research with multiple age groups, initiating research, choosing methods for data collection, and negotiating power relations and ethical practices.