Purpose: Typically developing multilingual children's speech may include mismatches and phonological patterns that are atypical in monolingual peers. One possible reason for mismatches is cross-linguistic transfer, when structures unique to one language are used while speaking another language. This study explored cross-linguistic transfer in Vietnamese-English-speaking children's and adults' speech at phoneme and syllable levels. Method: Children (n = 66) aged between 2;0 and 8;10 (years;months) and adult family members (n = 83) completed single-word speech assessments in Vietnamese and English. Cross-linguistic transfer of nonshared consonants was analyzed in terms of frequency, patterns, and direction of transfer. Syllable structure patterns in English consonant clusters (cluster reduction, cluster simplification, epenthesis) were identified, as consonant clusters do not occur in Vietnamese. Results: Cross-linguistic transfer of nonshared consonants occurred in most children's speech at least twice (75.76%, n = 66), tended to happen when the target was nonshared and occurred at a low frequency. During Vietnamese speech assessment, 21.82% of children's and 26.30% of adults' mismatches were due to cross-linguistic transfer of English consonants. During English speech assessment, 2.84% of children's and 24.33% of adults' mismatches were due to cross-linguistic transfer of Vietnamese consonants. Direction of cross-linguistic transfer was significantly associated with children's age and language proficiency. Bidirectional cross-linguistic transfer only occurred in the youngest children (2;6-4;5). English consonant clusters were impacted by syllable structure patterns for both children (35.76%) and adults (22.95%), indicating these mismatches in Vietnamese-English-speaking children's speech may be due to development, cross-linguistic transfer of Vietnamese syllable structure to English, and/or ambient phonology. Conclusions: Cross-linguistic transfer occurred in Vietnamese-English-speaking children's and adults' speech. These findings highlight the importance of assessing multilingual children's speech in each language, considering interaction between languages, and identifying reasons for mismatches to ensure accurate diagnosis.