The development of multimedia educational materials aimed at primary school students is of great importance today. The design, mainly of images and text, can influence the efficiency of these materials in the teaching-learning process. One of the most widespread theories that deals with the correct design of multimedia materials is the Mayer's Cognitive Theory of multimedia learning. In this research work we are interested in validating some of the principles enunciated by this theory in materials aimed at primary school students (children aged seven and eleven) and in the field of geometry teaching. For this purpose, we performed four experiments using the eye tracking technique in the data collection process. Using eye tracking, we can analyze how the process of the visual observation of multimedia contents is produced. The main contribution of this study is the proposal for the use of this evaluation method, which allows us to validate empirically and objectively (physiologically) aspects such as the comprehension process of multimedia content, the attention of the students while analyzing the information provided or the cognitive load of the supplied materials. Faced with questionnaires or surveys, which have been used traditionally to analyze the efficiency of multimedia materials and have a more subjective nature, eye tracking provides information not consciously controlled by the students. Its use is a particularly interesting and useful source of information in the case of children, allowing us to collect information on their interests and preferences, which is more difficult to obtain using traditional techniques.