When a new linguistic code emerges and stabilizes, what are the roles of children and adults in leading and consolidating the changes? This question lies at the intersection of child language acquisition and contact-induced language change. Adults and children have played different roles in the development of a new mixed code, Light Warlpiri, spoken in a Warlpiri community in northern Australia that arose from code-switching practices among bilinguals. Elements from typologically dissimilar languages are combined systematically in the new language, with verbal and nominal structures derived from different sources. Verbal morphology is from English/Kriol (which have fixed nominative-accusative word order patterns), with the addition of some innovations, probably brought in by speakers who were then children. Nominal case morphology is from Warlpiri (with ergative-absolutive case-marking, and flexible word order). But Light Warlpiri shows redistributions of case suffix allomorphy derived from Warlpiri. The paper shows the emerging case-marking patterns in Light Warlpiri, and tracks the roles played by children and adults in the changes.