Despite growing research on individual differences in child bilinguals, few studies have focused on the development of syntax, included both languages, and studied newly arrived school-age migrant children. Accordingly, this study investigated the syntactic development of heritage language (HL) Syrian Arabic and L2 English by Syrian refugee children (N = 119) recently arrived in Canada using a sentence repetition task. Regression analyses showed that a partially overlapping set of child-level (input and cognitive skills) and language-level (syntactic structure) factors accounted for performance in each language. HL performance was particularly sensitive to language, cognitive, and input variables indexing cumulative HL exposure. L2 performance, however, was sensitive to cognitive and environmental variables indexing current and cumulative L2 use. Finally, despite stronger performance in Arabic than in English, results revealed interdependence between the two languages, indicating that participants with stronger syntactic abilities in their HL tended to have stronger syntactic abilities in their emerging L2.