Research highlights the potential of interprofessional in-situ simulation (ISS); however it is often limited by individualistic outcome measures. Using an activity theoretical design, this research aimed to develop, implement and analyse an ISS programme intended to promote expansive learning and organisational change for paediatric emergencies in general practice. An interprofessional team participated in an adapted Change Laboratory intervention 'SimLab', which consisted of four sessions comprising two ISS and four facilitated group discussions. Audio recorded group discussions were transcribed and analysed using activity-theoretical analytical tools, the cycle of expansive learning, and the concept of contradictions. Participants mapped a learning process, from questioning their current practice to consolidation and generalisation of collaboratively designed new practices. This enabled the team to develop their collective expansive learning and adopt a systemic perspective during discussions, enhancing paediatric emergency care preparedness and facilitating organisational change. This research provides new knowledge on theory guided design and instruction applied to paediatric emergency care ISS training in general practice. The study provides tools to improve our understanding of how to pedagogically facilitate and enhance delivery of interprofessional ISS training to enable expansive learning. The SimLab design, is potentially transferrable providing exciting learning opportunities for other clinical settings and contexts.