With increasing focus on primary curricular reform in Ireland, there is a growing recognition of the significance of early childhood education. As a result, professional development organisations are now examining the effectiveness of science, technology, engineering and mathematics (STEM) education for young children. This study investigates how teachers in a multi-grade primary school initially approached teaching integrated STEM lessons to children aged 4-7 years. Over the course of one school year, three teachers engaged in four cycles of lesson study. Through an action research approach, the researchers collected qualitative data from interviews, lesson plans, collaborative weekly meetings and observation sheets. The study identified difficulties associated with the teachers' limited comprehension of integrated STEM education and demonstrated how the Engineering Design Process helped to mitigate these challenges. The findings offer valuable insights into the knowledge requirements involved in developing children's 21st century skills. The research concluded that teachers enhanced children's STEM learning by providing opportunities for them to explore their curiosity, make predictions and experiment through open-ended play experiences.