Previous research has established that children's experiences of language during in-person interactions (e.g. individual and cumulative experiences of structural choices) implicitly shape language learning. We investigated whether children also implicitly learn structural choices during online interactions, and whether this is affected by the visual co-presence of a partner. During an online conference call, three- and five-year-olds alternated describing pictures with an experimenter who produced active ("a cat chased the dog") and passive ("the dog was chased by a cat") prime descriptions; half the participants had video+audio calls, and half had audio-only. Children in both age groups produced more passives after passive than active primes, both immediately and with accumulating input across trials; neither effect was influenced by call format (video+audio vs audio-only). These results demonstrate that implicit grammar learning mechanisms, as evidenced by syntactic priming effects, operate during socially contingent online interactions. They also highlight the potential of online methodologies for developmental language production research.