The present chapter discusses the acquisition of social and linguistic constraints by Jakarta Indonesian preschoolers. We examine the morphological aspects in two Indonesian varieties, namely Bahasa Indonesia (BI) and Colloquial Jakarta Indonesian (CJI), focusing on the use of transitivity and intransitivity marking verbal prefixes. Data in this study come from two periods of interviews in formal and informal settings with a six-month interval. Participants in this study are children of middle-class families (N = 63). We investigate whether they use BI prefixes and CJI prefixes appropriately in terms of situations and morphological rules. Findings in this study indicate that these children acquire the social and grammatical constraints at the same time in the informal situation.