This study explores the ways in which young second language learners in an intact fourth-grade content and language integrated (CLIL) science class drew on the affordances of multiple semiotic resources including language, images, sound, movement, etc., to construct disciplinary knowledge in the context of a multimodal project on machines. After an instructional period in which attention to literacy was embedded in their science classes, the learners designed and illustrated their own complex machine, filmed a digital video and produced a written explanation. By integrating a systemic functional linguistics-informed analysis of the linguistic features of the children's written texts with a semiotic analysis of the intermodal relationships in their projects, we offer insights into the interrelation of different modes (linguistic, visual and aural) in facilitating children's meaning-making in the L2. The results revealed considerable within-group variability in the learners' written texts and intermodal relations of concurrence and complementarity in their multimodal projects. The role of multimodal tasks in providing all children with effective tools for communicating knowledge is discussed.