The instructional use of animals is a popular strategy to engage students with science, enhance their motivation, and promote values such as respect, tolerance, and empathy for all living beings. Although these beneficial outcomes are widely acknowledged, research has not provided reliable indicators of their efficiency. Therefore, it is essential to broaden the studies focused on the use of animals in education. In this regard, it becomes necessary to understand students' attitudes towards animals. This paper presents data on the attitudes of primary school children towards the humane treatment of animals. It follows the implementation of a longitudinal project based on the concept of "classroom pet", aimed at fostering the development of scientific reasoning competencies and positive attitudes towards animals. To assess the project's efficacy, a methodology combining quantitative and qualitative assessment approaches was outlined. The study involved 43 students, aged 8-10, from two fourth-grade classes of the same school. Findings concerning how children's attitudes towards different animals and animal uses are modulated as the result of an animal-based educational intervention and naturally throughout their maturation are discussed. This study provides relevant information for the development and evaluation of humane educational programmes. (Contains 1 figure and 2 tables.)