Disposable diapers contribute to the generation of plastic waste globally, with more than 8 million tonnes of diapers ending up in landfills or incinerated each year. One strategy for reducing this waste while maintaining child and parent health and well-being is to increase reusable diaper use and reduce delays in toilet training. We outline a protocol for applying behavioural science to understand the factors influencing these behaviours and to generate intervention recommendations. The research will be conducted in two stages. Stage 1 involves semi-structured interviews and surveys, guided by the Capability, Opportunity, Motivation-Behaviour (COM-B) model, to explore factors influencing reusable diaper use and toilet training at 18–30 months. Data will be analyzed using quantitative (regression and ANOVA) and qualitative (deductive and inductive) methods. Stage 2 involves triangulating findings to identify key barriers and enablers. Intervention frameworks (Behaviour Change Wheel and BCT Taxonomy) will help identify potential behaviour change intervention strategies to target key barriers and enablers identified within COM-B domains. Interventions will be presented to multidisciplinary stakeholders and nominal group technique process will be used to agree on the most promising, feasible, and acceptable options for development and piloting. The findings will help generate recommendations for supporting UK government policy and practice change in this area.