The increasing pervasiveness of voice input systems in consumer devices (e.g., Apple's iOS Siri) creates the potential for young children to use features and access content that previously required the ability to read and write. However, whether and how young children use voice input systems and associated voice agents on mainstream devices has not been studied in detail. This paper reports preliminary findings from an online survey with parents about children's use of voice input systems and a content analysis of YouTube videos depicting child interaction with one popular voice input system. Our results reveal three primary ways in which children use voice input systems: exploration, to understand and relate to the voice agent and for fun; information seeking, related to fact-finding questions and information about their surrounding environment; and functional, as a means of operating the device. While our results are preliminary, they highlight a variety of opportunities and challenges voice input systems present for children and parents.