This paper presents children's writing in their vernacular family language, Moroccan Arabic. It first provides some background to the family and school literacies of nine-year-old children of Moroccan Arabic (MA) background growing up in France with French literacy practices at school and Written Arabic literacy practices in the home. The paper then presents the results of an orthographic analysis of the children's attempts to write their vernacular family language, MA, an Arabic dialect not generally used for written purposes. When writing MA for the first time, the children have to rely on the implicit and explicit knowledge of the writing systems they have already acquired, namely, French, the language of schooling, and Standard Arabic which is regularly present in the home, although the children's competence in it is limited. The children's writing in Moroccan Arabic is highly structured and shows how the children harness French orthographic rules to conceive the new writing system.