Rare and undiagnosed diseases collectively represent a global health priority, presenting distinct challenges for healthcare systems due to their low prevalence, cumulative frequency, and complex care requirements. The impact of rare and undiagnosed diseases on children and their families extends beyond physical and mental health, affecting every aspect of their lives. This paper outlines the development of an innovative Model of Care that emphasises cross-sector care coordination as an approach to enhance the health and wellbeing of Western Australian children living with rare and undiagnosed diseases. Detailing insights gained will support and guide healthcare professionals to create services that improve outcomes for people living with rare diseases and undiagnosed diseases and their families.