This study used a brief vocabulary training paradigm to examine two factors for cross-language transfer: how similar the first language (L1) is to the second language (L2) and L1-L2 proficiency levels. Fifty-four sequential bilingual children (aged 6–8) with similar L2 English proficiency levels were assigned to three equal groups: a Vietnamese-English group with low L1 proficiency, a Spanish-English group with low L1 proficiency, and a Spanish-English group with high L1 proficiency. Individual training consisted of two mediated learning experiences conducted in the L1 targeting eight vocabulary items using narrative-based activities. Four of eight target words were cognates between Spanish and English. Pre- and posttesting measured definition quality in the L1 and L2. All groups showed improvement in the L1 following training, but only the Spanish-English group with high L1 proficiency improved in L2 English, revealing a degree of spontaneous cross-language transfer among children with high L1-L2 proficiency.