Giving practitioners a voice is essential in developing intervention programs that are adapted to educator and child needs. We aimed to do so by involving educators in the codesign of an intervention supporting early mathematical and executive development, to maximize feasibility and implementation quality. N = 100 educators, N = 24 Early Years Centers (EYCs), and N = 16 education science experts took part. Educators expressed a need for feasible professional development coupled with practical activity suggestions (Phase I). A scientist/educator expert panel developed the intervention (Phase II), further refined via educator input (Phase III) to overcome implementation barriers (e.g., diverse children's needs). In Phase IV, mixed methods revealed successful solutions (e.g., differentiation guidance) and additional barriers (e.g., diverse staff demands). We demonstrate the value of an iterative codesign process for complex educator-led interventions to produce programs addressing specific needs of educators and children whilst retaining key theoretical principles.