This study investigated how various child-internal and child-external factors predict English L2 children's acquisition outcomes for vocabulary size and accuracy with verb morphology. The children who participated (N=169) were between 4;10 and 7;0 years old (mean = 5;10), had between 3 to 62 months of exposure to English (mean = 20 months), and were from newcomer families to Canada. Results showed that factors such as language aptitude (phonological short term memory and analytic reasoning), age, L1 typology, length of exposure to English, and richness of the child's English environment were significant predictors of variation in children's L2 outcomes. However, on balance, child-internal factors explained more of the variance in outcomes than child-external factors. Relevance of these findings for Usage-Based theory of language acquisition is discussed.