This longitudinal study focuses on the language production of two siblings, aged 6 and 9 at the beginning of the data collection period, who have been brought up in a bilingual family in New York. The study specifically examines the phenomenon of code switching, or transitioning between languages. I studied the extent to which these switches are caused by deficiencies in vocabulary in a specific language, and seek to identify other possible causes for such transitions. The data collected mainly through recording and transcription of the children's speech within the family home allowed me to identify a number of sociopragmatic functions most commonly fulfilled by producing mixed utterances. The distribution in the amount and function of code switches turned out to be in a dynamic state, with both quantitative and qualitative changes observed throughout the study period. I conclude that code switching, especially in the case of child speech, should be considered a fluid and multifaceted phenomenon which represents the speaker's role in the conversation and reflects multiple social and pragmatic functions.