Current models of the affinity between syntax and vocabulary are complex and recognize the contribution of bootstrapping and computational processes. To date, the mutual facilitation between these two constructs over time has not been studied in second language (L2) school children. The present study investigated longitudinally the direction and strength of the associations between syntactic and vocabulary skills in English Language Learners (ELLs; N = 409, 204 females, M age = 78 months, low socioeconomic status backgrounds) and their monolingual English-speaking peers (EL1; N = 157, 92 females, M age = 77 months, low socioeconomic status backgrounds). Children were assessed annually from Grade 1 to Grade 4 on a syntactic and a vocabulary task. Overall, autoregressive cross-lagged analyses indicated that early syntax predicted later vocabulary and vice versa, yet, the magnitude of prediction varied across groups. Notably, in the early stages of L2 learning, the predictive power from vocabulary to syntax was stronger than that in the opposite direction. Moreover, the predictive power from vocabulary to syntax was consistently stronger in the ELL than in the EL1 group. The results suggest that, in general, with sufficient quantity and quality of exposure to the L2, lexical and syntactic bootstrapping coexist. However, among novice young ELLs, bootstrapping is stronger from vocabulary to syntax than the other way around. Results underscore the importance of studying the relations between vocabulary and syntax longitudinally, and caution about an injudicious application of L1-based models to young L2 children's language development.