BACKGROUND: Developing theoretical courses for post-graduate medical training that are aligned to current workplace-based learning practices and adaptive to change in the field is challenging, especially in (sub) specialties where time for re-design is limited and needs to be performed while education continues. APPROACH: An instructional design method was applied based on flexible co-design to improve post-graduate theoretical courses in child and adolescent psychiatry (CAP) in the Netherlands. In four phases over a period of three years, courses were re-designed at a national level. EVALUATION: Once common vision and learning goals were agreed upon and the prototype was developed (phases 1 and 2), the first courses could be tested in daily practice (phase 3). Phase 4 refined these courses in brief iterative cycles and allowed for designing additional courses building on and adding to previous experiences in brief iterative cycles. The resulting national theoretical courses re-allocated resources previously spent on a local level using easily accessible online tools. This allowed trainees to align content with their clinical rotations, personal preferences and training schedules. REFLECTION: The development of theoretical courses for post-graduate medical training in smaller medical (sub-)specialties with limited resources may profit from a flexible instructional design method. We consider the potential merit of such a method to other medical specialties and other (inter-)national efforts to develop theoretical teaching courses. A longer-term implementation evaluation is needed to show to what extent the investment made in the re-design proves to be future-proof and enables rapid adaptation to changes in the field.