Writing is an important literacy skill for K-12 students' academic success. For English as a Second Language (ESL) children, developing writing skills involves both learning English and learning to write. This makes ESL writing instruction challenging as teachers have to strike a balance between teaching writing as a literacy skill and as a tool for students' English language development. Recent research has identified that in-service teachers in K-12 settings lack requisite training in L2 writing, resulting in various challenges in the ESL writing classroom. One such challenge for them is to determine whether the focus of writing instruction should be to teach students how to write (learn-to-write) or to utilize writing as a tool to help students develop the English language (write-to-learn language). Eliciting the theoretical orientations of both learn-to-write (LW) and write-to-learn language (WLL), this article suggests that the LW and WLL approaches are not mutually exclusive for teaching ESL writing. Based on a review of recent research, the paper discusses a systemic functional linguistics (SFL)-informed genre-based writing pedagogy as well as teaching and learning activities that integrate both LW and WLL principles into ESL writing instruction in the elementary classroom.