Learner corpus studies using syntactic complexity as a construct for characterizing learner proficiency have found that higher proficiency permits learners to produce more complex syntactic structures. However, the majority of previous studies have focused on writing, almost exclusively with adult second language (L2) learners. Given the fundamentally different mechanisms underlying speaking and writing activities, this study investigated (a) how different processes involved in writing and speaking performance affect child L2 learners’ sentence production, and (b) whether syntactic complexity is a better predictor of proficiency in a particular production type. To this end, we analyzed syntactic complexity for written and spoken corpora supplied by native Korean-speaking child learners of English using 7 syntactic complexity indices. Results showed that learners used longer sentences, more subordination, more verb phrases per T–unit, and less coordination in writing than in speaking. In addition, a prediction model fitted to the written corpus explained more of the variance in proficiency scores than a model based on the spoken corpus. These findings indicate that the different processes underlying writing and speaking influence the way that beginning-level child L2 learners produce sentences in writing and speaking tasks.