A close relationship between children's vocabulary size and the grammatical complexity of their speech is well attested but not well understood. The present study used latent change score modeling to examine the dynamic relationships between vocabulary and grammar growth within and across languages in longitudinal data from 90 simultaneous Spanish–English bilingual children who were assessed at 6‐month intervals between 30 and 48 months. Slopes of vocabulary and grammar growth were strongly correlated within each language and showed moderate or nonsignificant relationships across languages. There was no evidence that vocabulary level predicted subsequent grammar growth or that the level of grammatical development predicted subsequent vocabulary growth. We propose that a common influence of properties of input on vocabulary and grammatical development is the source of their correlated but uncoupled growth. An unanticipated across‐language finding was a negative relationship between level of English skill and subsequent Spanish growth. We propose that the cultural context of Spanish–English bilingualism in the US is the reason that strong English skills jeopardize Spanish language growth, while Spanish skills do not affect English growth. A video abstract of this article can be viewed at: https://youtu.be/qEHSQ0yRre0 Young bilingual children's rate of growth in vocabulary and grammar are related within English and within Spanish (shown here for English). There is no evidence that grammar depends on vocabulary or vocabulary depends on grammar, nor do the relations seem due to a general ability because they are strong and reliable only within languages. We suggest the tandem development of vocabulary and grammar reflects the influence of properties of input that benefit children's development in both linguistic domains, but only in that language.