This study examined the code-switching patterns in the speech of immigrant Spanish-speaking children. Seven- and 10-year-old boys and girls from bilingual classrooms were each paired with a mutually selected friend, and their speech was collected in two contexts: while the children waited for an expected science experiment and when they worked together to follow an instruction worksheet about hands-on magnetic materials. This study presents data on the discourse characteristics of children's code switching, and the functions that Spanish and English have according to context. In addition, the data are described in relation to children's language competence and preference. The study found that code switching occurred both within and across turns. The older children's switches were more frequent and were deployed for a wider variety of functions than the younger children's. The results challenge the negative view that code switching by children who are learning two languages is due to lack of proficiency, and instead support the view that it is used as a strategy to extend their communicative competence during peer interaction.