Generalisation processes belong to the core of algebraic thinking. The development of these processes begins as early as the primary school classroom, when children describe and reason about mathematical patterns. Our theoretical framework includes a distinction between patterns and structures. The focus on structures is important for children to engage in reasoning about patterns rather than just describing them on the surface. In an exploratory study with 45 third and fourth graders, we use an epistemological perspective to examine how this distinction is reflected in children's generalisations. The results create awareness of the algebraic quality of justifications.