In this article we aim to deepen our understanding of the content and nature of the early childhood teacher's knowledge, focusing on those aspects which might promote students' algebraic thinking. Approaching arithmetic from the viewpoint of algebra as an advanced perspective and considering the analytical model "Mathematics Teachers' Specialised Knowledge," we analyse the specialised knowledge in a classroom of five-year-olds handled by an experienced teacher in a lesson on the decomposition of the number 6. Moreover, alternative management of the session is proposed in order to promote early algebraic thinking. In the domain of "mathematical knowledge," this analysis has revealed the specificity of the knowledge that this professional must have of the natural number. In the domain of "pedagogical content knowledge," it has highlighted the many elements of the "knowledge of mathematics teaching" that should be possessed to promote algebraic thinking at this educational stage. These elements appear to be more closely related to a profound knowledge of the mathematics taught than to pedagogical knowledge of a more general nature. [Translation from English by Mercè Rius.]