Aims: We investigated the extent to which child heritage speakers of English in contact with Spanish in Mexico accurately produce phrasal verbs, and if not, whether their difficulties are modulated by chronological age, language dominance, and experience. Methods: We implemented an elicited production task to elicit the use of idiomatic and transparent phrasal verbs in school-age children using a cross-sectional design. We tested 26 English-heritage children and adolescents (7.5–17.3; M = 10.4) residing in Mexico. Their results were compared with those of 18 English monolingual children born and raised in the American Midwest (6.11–14.9, M = 10.2). Chronological age, dominance, and patterns of language experience were included as covariables. Data and Analysis: Data were analyzed using generalized linear mixed models with binomial distribution. Findings: The heritage children exhibited significant difficulties in accurately using phrasal verbs compared with monolingual children, particularly with idiomatic phrases, revealing an asymmetry between the two structure types. Difficulties were modulated by age and linguistic experience. Importantly, as children aged and experienced extended exposure to English, the observed challenges diminished, aligning with recent findings in heritage language development. We posit that these divergences are attributed to protracted development during childhood. Originality: This study expands on recent work with adult heritage speakers of English by tracking the development of phrasal verbs in school-age children quasi-longitudinally rather than observing the adult-like outcome. Furthermore, examining young and older school-age heritage speakers of English in contact with Spanish as the dominant language in Mexico contributes to existing research primarily centered on heritage speakers of minoritized languages in the United States.