Tools to measure perceptions of child wellbeing using a questionnaire are commonplace in educational research. However, the design and development of these tools vary widely. This case study considers a new approach used by researchers who were part of a collaborative, mixed-methods study of child wellbeing, resulting in Australia’s first nationally representative and internationally comparable online questionnaire of child wellbeing in the middle years of schooling. The researchers, expert in quantitative methods, worked in partnership with social policy researchers, experts in qualitative research methods, to develop a large-scale questionnaire of child wellbeing. The questionnaire design was informed by children’s perspectives on their own wellbeing using qualitative research methods. In particular, the perspectives of children in Australia who may experience disadvantage or marginalization were gathered. Attention was also given to ensuring that questionnaire administration was flexible, interactive, age-appropriate, and sensitive to children’s needs. Furthermore, the questionnaire needed to be quantitative in kind, internationally comparable with other questionnaires, and self-completed online by students in Years 4, 6, and 8 in schools across Australia. This case study presents some of the methodological challenges and opportunities in large-scale questionnaire design and online implementation as well as learnings that may be useful for educational researchers and tertiary students undertaking or working in the field of questionnaire research.