This article report the principles and the design of usability method used on HMK-Learning application. It is a digital tool for students at primary schools in France based on Mounier's numeracy work in didactics. Firstly, the design method of HMK-Learning is described. Then, we describe the survey design method to study the usability of HMK-Learning. This questionnaire is based on Nielsen's and Alsumait's heuristics. Next, we present the survey results answered by four teachers of primary schools. We mainly report them from the point of view of the design interface elements. The study underlines the importance of offering interaction and feedback techniques adapted to the motor and cognitive abilities of elementary school students.