Argumentation is a fundamental communicative ability that children develop over time through formal schooling and daily practice with peers and family members. Literature on children's argumentation appears to have focused on their social interactions out of school, clinical environment, or informal pedagogic contexts. Even though there are research inquiries into children's argumentation in formal academic learning, many have been focused on argumentative writing in math or science classes. Much less is known about teacher-led argumentation and the youngest children's emerging argumentation in language art classes, where argumentation is formally and systematically introduced and learned. This paper reports a year-long ethnographic study on argumentation in a first-grade English language art classroom in the United States. Ethnographic discourse analysis was conducted to analyze two key literacy events from the daily reader's and writer's workshop. It is supplemented with qualitative analysis of the researchers' field notes and the students' artifacts. Our findings highlight the inherent intertextual nature of children's argumentation and a critical role the teacher played in eliciting and steering the children's argumentation construction through strategic instructional conversations (especially accountable talk). Our findings also revealed teacher-led children's intertextual argumentation as a powerful heuristic process and tool to enrich students' learning. The paper concludes some classroom argumentation teaching practices based on the research findings.