This article focuses on professional development among teachers within a Swedish national network on toddler mathematics education. The activities within this network can be understood as participant-oriented collective professional development based on a knowledge of practice approach. An inventory of toddler mathematics was performed within this network through observations of authentic mathematical activities. The inventory made diversity in the participating teachers' perceptions of toddler mathematics visible; different activities were labelled similarly and opposite, similar activities were labelled differently. The inventory made clear the need for theoretical influence and discussions in practical inquiry. The addition of theoretical notions made previous invisible diversity among the network members visible, and strengthened the opportunities for further collective professional development. Diversity becoming visible contributed to the knowledge of practice approach, as well as the development of the professional language of the participating teachers.