This paper describes an iterative participatory curriculum design approach to developing a problem-based STEM curriculum for preschool children. The curriculum aims to teach young children problem-solving using an adapted version of the engineering design process (EDP). Despite evidence showing that a rigorous, integrated STEM curriculum promotes cognitive development and curiosity, very little STEM or engineering instruction occurs in classrooms for three- to five-year-old children, and few studies include teachers in the curriculum design process. Research has shown that, when children experience an engineering curriculum, they show an increase in engagement, in the number of engineering behaviors displayed, and in persistence in completing activities. As well, when teachers are involved in designing curriculum, they are more likely to feel empowered and sustain implementation. Qualitative analysis of semi-structured interviews with 13 preschool teachers after the development process showed that teachers who participated in the process perceived increased knowledge and self-efficacy in teaching STEM in their classrooms. These reflections support using a participatory curriculum design approach for empowering teachers and enhancing self-efficacy in teaching STEM to young children. High teacher self-efficacy has been associated with positive classroom outcomes and teacher retention in the profession.