Mathematics education is a critical part of the curriculum for students worldwide (Linder, Costello & Stegelin, 2011). Children possess some mathematical understanding before entering into formal schooling. However, pre-primary education provides them with a platform where they start their systematic learning of mathematical concepts by connecting them with their daily life experiences. In this connection, effective teaching and learning processes make a difference in young children’s attitude towards mathematics and build a strong foundation for mathematics learning later in life. The effectiveness of teaching depends on the teachers’ content and pedagogical skills, which are enhanced through different courses and collaborative work with other teachers. Action research is one of the effective methods for continuous professional development of teachers as it informs teachers about their practices and empowers them to take leading responsibilities in their own teaching contexts (Donato, 2003). In order to facilitate pre-primary teachers’ and my own professional skills particularly in developing number sense in young children, I conducted a collaborative action research in one of the private schools in Karachi. Two pre-primary teachers and four pre-primary students participated in the study. Collaborative planning, teaching, interview, reciprocal observation of teaching methods and informal discussions with participant teachers helped me to collect and enrich my data. The data indicates that the collaborative action research benefitted both teachers and the young learners in multiple ways. It benefitted teachers in developing pedagogical skills through planning, implementing, reflecting and re-planning with necessary changes in a cyclic way. It helped us in learning both skills and content from each other’s experiences and teaching practices, through co-teaching and providing feedback to each other. The study was helpful for young children to develop their understanding of numbers through their active engagement with concrete materials for their exploration. It also enabled them to learn mathematical concepts in a smooth transition from concrete to representational to abstract level. The results of this study can be replicated in a public school context and in a rural setting over a longer period of time. On the basis of the study findings, I believe that this study can help pre-primary teachers and teacher educators in identifying different strategies and activities to enhance young children’s mathematical understanding.