This article report the principles and the design of HMK-Learning, a digital tool for students at primary schools in France. The design is based on Mounier's numeracy work in didactics. Then, we describe the questionnaire design method to study the usability of HMK-Learning. Next, we present the questionnaire results answered by four teachers of primary schools. Finally, we discuss them from the point of view of the design of HMK-Learning and the pedagogical questions they raised. This paper also highlights the highly important role of didactics in the process of designing a digital tool for numeracy learning for primary school children that we have identified in the answers of the usability questionnaires.