In South Africa and in many other parts of the world, decolonising the curriculum has become a valued goal, while frameworks that would systematically support the decolonising project through "instructional design" are not broadly available. In this conceptual paper we bring readers to consider one framework for instructional design, the theory of Realistic Mathematics Education, and discuss how it can aid in decolonising education in primary years. We exemplify our conceptual position through an instructional design for teaching length measurement--a key grounding practice in both early years mathematics and science throughout the world. We propose that the resulting decolonised narratives are not only useful in marginalised contexts of countries with a history of colonisation. Due to their positioning of mathematics as a human endeavour, these narratives are capable of generating meaningful, equitable engagement with mathematics for diverse student groups in a variety of educational settings.