Background Despite the popularity of the theory of planned behaviour (TPB), a lack of research assessing the efﬁcacy of the model in understanding the health behaviour of children exists. The aim of this study was to develop and test a TPB-based measure suitable for use with schoolchildren aged 9–10 years.Methods A mixed-method sequential design was employed. In Phase 1, semi-structured focus group discussions were conducted to elicit the underlying beliefs speciﬁc to tooth brushing. Using content thematic analysis, the beliefs were identiﬁed and a TPB measure was developed. A repeated measures design was employed in Phase 2 using test–retest reliability analysis in order to assess its psychometric properties. In all, 184 children completed the questionnaire.Results The questionnaire proved to be reliable for assessing the tooth brushing beliefs of children. Pearson’s product moment correlations were calculated for all of the TPB constructs, achieving substantial to almost perfect agreement levels, providing strong predictive power of the TPB.Conclusions The design and development of the measure are described, which could serve as a guide to fellow researchers and health professionals interested in using theoretical models to investigate the health and well-being of children.