Cross-linguistically, monolingual children produce target-like inflected verbs much earlier than they can reliably distinguish between singular and plural subject-verb agreement morphology in comprehension (i.e. Johnson, V., J. de Villiers, and H. Seymour. 2005. "Agreement Without Understanding? The Case of Third Person Singular /s/." "First Language" 25: 317-333; Pérez-Leroux, A. T. 2005. "Number Problems in Children." In "Proceedings of the 2005 Annual Conference of the Canadian Linguistics Association," edited by C. Gurski, 1-12. https://cla-acl.artsci.utoronto.ca/wp-content/uploads/actes-2005/Perez-Leroux.pdf). In heritage speakers, Spanish agreement morphology shows optionality due to reduced input, especially when children transition to English schooling (i.e. Montrul, S. 2004. "The Acquisition of Spanish: Morphosyntactic Development in Monolingual and Bilingual L1 Acquisition and Adult L2 Acquisition." Amsterdam: John Benjamins; Jacobson, P. 2012. "The Effects of Language Impairment on the use of Direct Object Pronouns and Verb Inflections in Heritage Spanish Speakers: A Look at Attrition, Incomplete Acquisition and Maintenance." "Bilingualism: Language and Cognition" 15 (1): 22-38). We investigate how simultaneous heritage bilingual children's interpretation and use of subject-verb agreement in English and Spanish may be modulated by increased input and activation of Spanish in dual language education (DLE). 125 participants aged 3-7 (42 heritage children, 40 English monolinguals, 39 Spanish-dominant children) took part in a fill-in-the-blanks task and a picture-matching task. In English, bilingual comprehension accuracy surpassed that of the monolinguals. In Spanish, bilingual production lagged behind that of the Spanish-dominant children and language "output" was found to be a greater predictor of productive accuracy than increased activation of Spanish in DLE. The implications of these results for theories of bilingualism and DLE are discussed.