Although education research in critical thinking is a rapidly emerging field, how teachers can facilitate, monitor, and support critical thinking in primary science remains a challenge. This paper builds on recent semiotics mediated scientific reasoning research to present a fresh perspective of critical thinking in primary science. Analysis of student and teacher interactions in an innovated semiotics grounded learning design and pedagogy demonstrates a recursive process of collaborative construction, reviewing and refinement of representational systems and models. The multimodal epistemic practices of the discipline, involving the generation, evaluation and refinement of the semiotic systems that frame scientific ideas and practices are presented to both stimulate and support generative critical thinking. How semiotic systems in primary science can provide a focus, stimulus, and tool, for not only students' generative critical thinking but also teacher monitoring and support is presented.