Languages differ in how they encode causal events, placing greater or lesser emphasis on the agent or patient of the action. Little is known about how these preferences emerge and the relative influence of cognitive biases and language-specific input at different stages in development. In these studies, we investigated the emergence of sentence preferences to describe causal events in English- and Japanese-speaking children (aged three and five years) and compared this to preferences displayed by adults. We studied two factors suggested to influence this choice: Language (Corpus study & Experiment 1) and Intentionality (Experiment 2). Participants watched videos depicting familiar and novel causal actions, and made a best-match choice between a transitive and intransitive description. We found a stronger preference for intransitive sentences with causal verbs and more frequent argument omissions in Japanese child-directed speech than in English child-directed speech. The trajectory of acquisition in the selection of transitive sentences for causal events differed between languages. For intentionality, with familiar verbs both Japanese and English speakers selected fewer transitives for accidental than intentional scenes, but this pattern was more pronounced in Japanese speakers. However, with novel verbs, only five-year-olds and adults showed this preference. These data provide important new information to constrain theories about the process of learning to map event structure to language, and its interdependence with intentionality and the distributional properties of linguistic input to children.