The Mother Tongue-Based Multilingual Education in the Philippines requires children to learn the mother tongue, in addition to Filipino and English, in elementary school. Acquiring Filipino as a second language (L2) from a first language (L1) background is not a major challenge for most Filipino children since almost all Philippine languages share the ergative actancy structure. Speakers of accusative Chabacano in Zamboanga City show an exception. In grammaticality judgments and picture description tasks, fifty 7-8-year-old L1 Chabacano learners overgeneralized case marking patterns in Filipino, revealing cross-linguistic influence. An SLA model for bilingual groups of learners with typologically different L1 and L2s argues for ‘grammatical consciousness-raising’ particularly within the first three grades in the elementary. Target grammatical structures can be taught implicitly and explicitly through adequate and meaningful classroom and parental language input within the threshold at which learners have already developed some degree of ability to use the L2.