Existing studies have demonstrated how children compose, experiment and use their imagination within the conventions of the tonal idiom with functional harmony. However, one area of research that has hardly been explored is how tonality emerges in the compositions of children who compose by transforming their own non-musical ideas, such as their drawings and stories. To that end, and within a context in which children individually compose at the piano, the present study examines the differences or similarities that, with respect to the tonal idiom, result from both approaches to composing. The findings demonstrate how interdisciplinary and symbolic thinking can help children achieve a better understanding of the tonal idiom and at the same time lower the threshold to compose.