There is a growing consensus in research that children’s numerical competencies start to develop at a very early age. However, less is known how toddlers (1–3-yearolds) learn about numbers and few tools are developed to make their progress in learning visible and researchable. In this methodological paper, we present the process of designing such a tool to be used in a combined research-development project. The focus of the paper is on the process of designing the tool that is based on theoretical principles, founded in the preschool traditions and attract young children’s attention.