Social sustainability embraces literacy development as a means by which children integrate their knowledge in society and become powerful and meaningful. In this context, the development of writing among young children requires the design of new teaching strategies that allow for the multimodal repertoire brought by children into the classroom. Systemic Functional Linguistics offers tools for the analysis of children's multimodal writing, which plays an important role in their literacy development. Our research was carried out in an urban context, the participants being 12 children aged 7 to 8. Data were collected through participant observation, conversations, and the analysis of documents and products generated by the children. From them, we analysed two stories written by two girls, which showed the way in which the children created meaning by combining verbal and visual modes, and how these modes interact (intersemiosis). The performance of a literacy task in which children are able to integrate their knowledge and heritage into the classroom, may constitute an interdisciplinary tool for their participation and engagement in the school, thus leading to a more equal society. In consequence, we propose that the integration of a genre-based pedagogy in the classroom should include greater awareness in teachers of the value of pupils' multimodal assessments.