Children represent a potentially crucial user segment for conversational interfaces. Computer systems interacting with children need to be tailored for these users so that they will understand child intent and so that the child will have a positive and successful experience with the system. This study focuses on discourse analysis of spoken-language child- machine interactions. In particular, politeness and frustration markers were analyzed using a database of child-machine conversations obtained from 160 children using a computer game in a wizard-of-Oz set up. Results indicate that younger children less likely to use overt politeness markers and more polite information requests compared to the older ones, with no apparent gender differences. Younger children, on the other hand, expressed frustration verbally more than the older ones; furthermore, frustration language was more predominant in male children.