The digestive system (DS) is a fundamental topic in biological science teaching. However, the literature indicates that students have difficulties in learning about it. In the present work, we focus on how early childhood Pre-Service Teachers (PSTs) develop their understanding of the DS regarding the CMP (Components-Mechanisms-Phenomena) framework of systems thinking. A teaching sequence was designed, implemented and iteratively improved over the course of three years, and in Year 3 included the construction of a physical model and the design and performance of a role-play. Data collection was performed using individual questionnaires before and after participating in the practical activities. The physical models and the role-plays were also analysed. The results show that participation in the sequence improved the understanding of the DS in all dimensions of systems thinking, especially in Year 3. The construction of physical models mainly fostered learning Components and the role-play seemed to facilitate a deeper understanding of Mechanisms. It is concluded that the combined use of both modes of representation constitutes a valuable strategy for science education.