The theorization of multimodality in academic scholarship is disconnected from how it is conceptualized by children. To bridge this gap, we analyzed 75 interviews with children about their digital video making. Analysis of their responses demonstrates children's socially-embedded, age-specific understandings of how modes operate, as well as when and why to employ them. In many cases, children's ideas ran counter to formal semiotic grammars and metalanguages of design. Bridging Systemic Functional Linguistics and social semiotics approaches with work in transliteracies, we argue for the need to advance age-centric social semiotic theories that center children's voices, purposes, and capacity to generate theory.