This study investigates how collaborative, collective mathematical reasoning emerges when students jointly solve problems regarding fair sharing. Three 6-year-old students worked collaboratively with three problems related to fair sharing, and their teacher was present. The data, captured in video recordings, was analysed using two frameworks: collective mathematical reasoning and the theory of joint problem space. The results are presented through four themes, each contributing to the problem-solving space and students’ possibilities to engage in collaborative collective mathematical reasoning. The themes are physical artefacts aimed at sharing resources, challenges related to the nature of the task, students’ conceptions of mathematics, and the teacher’s contributions, each playing different roles depending on where in the reasoning process they appear.