Formative assessment is an important driver in supporting children's writing development. This paper describes a writing rubric designed for use by teachers to formatively assess the writing of children in Pre-K to Grade 2, how the rubric was received by teachers, and its implementation in classrooms. Writing samples from 337 children in 33 classrooms in 7 schools in the Write to Read literacy improvement project were scored on conventions, organisation, ideas, word choice and voice. Agreement among raters was good as overall weighted Kappa values at each grade level ranged from 0.62 to 0.80. Confirmatory factor analysis supported three- and five-factor models. Coaches endorsed use of the rubric for providing formative feedback to students, identifying learning needs, and differentiating instruction. They highlighted how the rubric provides a framework through which teachers and students engage with the language of writing assessment and raise expectations about writing quality.