While generative AI has enhanced digital multimodal composing (DMC) processes, growing concerns about learners’ overreliance on AI highlight the need to foster critical AI literacy. This study first designed an instructional model based on theoretically informed components of critical AI literacy and then implemented it to explore how learners’ AI use and perceptions evolved during AI-assisted DMC. Data were collected from two high school-aged participants, including screen recordings of their DMC practices before and after the intervention, stimulated recalls, and semi-structured interviews. Findings show that both participants developed more critical use of AI tools, such as cross-validation of outputs, strategic prompting, and more independent composing. Their perceptions also reflected a more balanced redistribution of agency and authorship, as they selectively used AI tools to cope with technical and linguistic challenges. The study suggests that critical AI literacy can be meaningfully developed when process-based and critical AI literacy-informed instructional intervention is provided.