<p>This qualitative study explored how STEM learning and social emotional learning (SEL) were enabled with four and five-year-olds in a public after-school program in Northern California. Two teachers implemented a project and play-based pedagogy that used LEGO materials to guide the students in designing and building LEGO objects and structures and in programming and building LEGO robotic devices. The teachers also focused the program on developing SEL abilities. Eight of 11 students were in transitional kindergarten (TK) and three were in kindergarten while also participating in this free program. Researchers observed, video recorded, wrote video content logs and fieldnotes, reviewed curricular materials, and conducted semi-structured and informal interviews with the teachers during Fall and Spring semesters of one academic year. Results were that all students developed foundational science and math skills that were focused on in the lesson plans of a LEGO kit that the teachers used to guide students to design and build objects and structures. Further results were that the students developed technology and engineering computational skills and mindsets through the teachers’ use of lesson plans in a second LEGO kit to guide students to program and build robots. Finally, through the teachers’ extensive focus on SEL in conjunction with STEM learning in a project and play-based pedagogy, the students embodied and demonstrated the abilities and skills reflected in the five CASEL pillars. These findings are significant for informing the current movement in California and other states about viable pedagogical approaches and tools to stimulate and guide STEM learning at the TK and kindergarten levels, particularly as it can be facilitated through SEL.</p>