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 re-test 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 wellbeing of children.