The acquisition of part-whole understanding is considered fundamental in early mathematical education. However, recent research indicated that analogue and digital learning environments largely fail to offer systematic approaches to facilitate part-whole understanding. Therefore, we are currently developing a digital learning resource (DLR) that provides a systematic approach to learning part-whole relations. The aim of this article is to collate experiences made during the development process to derive lessons learned from a mathematics education perspective. Therefore, we chose the Design Thinking approach as a theoretical framework against which we reflected the iterative development process considering aspects important from the perspectives of mathematics education as well as software development. After completing a cycle of Empathizing, Defining, Ideating and Prototyping, the DLR received positive feedback in a first Testing phase. Throughout the development process, a major experience was that we underestimated programming workloads. Thus, our primary lessons learned were to (1) be aware of software development demands, (2) be open to the perspectives and needs of the other disciplines, and, (3) use existing tools allowing low-threshold communication throughout.