This study concerns children's representational knowledge, more specifically, their ‘invented notations’ of music. A small-scale empirical study of four 5-year-old children and their teachers working on the representation of music is reported. The challenges posed by the teachers and how the children respond to these challenges are analysed. The teachers challenge the children to explain their understanding and use contrast to direct children's attention towards distinctions and important terms in the domain of music. The children use coloured geometrical shapes on paper and a sequence of building blocks to represent music. By means of these visuospatial representations, sounding and conversing about them, the children are able to communicate their understanding of the relationship between representation (sign) and sound. The role of external representations in the development of children's musical knowledge is discussed.