Teachers' pedagogical practices have been consistently highlighted either as a support, or as a barrier, to girls' science, technology, engineering, and mathematics (STEM) engagement. Research suggests that teachers can display forms of gender bias during STEM experiences, such as unintentionally favouring boys in STEM participation. Investigation into early childhood and primary STEM education highlights teacher professional development as a priority, with a key focus on the awareness of gender issues. This study explores whether and how the STEM practices of early childhood teachers change and develop towards supporting girls' STEM engagement through Conceptual PlayWorlds as a model of practice. Through an educational experiment, the study design involved professional development workshops introducing teachers to the Conceptual PlayWorld model for the intentional teaching of STEM, with ideas on how to engage girls through the five characteristics of a Conceptual PlayWorld. Interview responses of seven teachers and one assistant principal regarding their experience of teaching STEM through Conceptual PlayWorlds are analysed in line with the cultural-historical approach. Findings highlight the teachers' developing consciousness regarding girls' STEM engagement through participation in the professional development workshops and Conceptual PlayWorlds, creating a shift in their teaching practices towards becoming more agentic in actively engaging girls with STEM. It is argued that while increasing teachers' consciousness informed and transformed their STEM practices, leading to teacher development, further relevant and meaningful professional development opportunities are required to support early childhood teachers in positioning STEM as a truly welcoming and inclusive space for girls.