This study examines the English vocabulary development of 43 very young child English as a foreign language (FL) learners (age 3.2-6.2) in China. They were tested twice for vocabulary breadth (reception and production) and semantic depth (paradigmatic and syntagmatic vocabulary knowledge). The development of the English vocabulary knowledge between these two measurements was predicted using a series of internal factors and external factors. An exploratory mixed-effects regression analysis revealed that English use, interacting with age of English onset, significantly predicted such growth. Older children benefitted more from practicing English to enhance their English vocabulary. The amount of English input at school was found to positively impact the development of English syntagmatic knowledge. Chinese paradigmatic knowledge significantly influenced the growth of English paradigmatic and syntagmatic knowledge. These findings indicate that at least at an early stage, external factors play an important role in child FL vocabulary development, particularly in children with a later age of English onset. Furthermore, the transfer of concepts from the first language to the second language (L2) might be more pronounced with respect to L2 semantic depth than L2 vocabulary breadth. This conceptual transfer is relevant to FL learners as young as three years of age.