Virtually all children who grow up bilingually code-mix at some time. Code-mixing is the use of two languages in a single unit of discourse, such as an utterance or a conversation. Code-mixing includes both intra-utterance mixing, or the use of languages in a single utterance (e.g., the utterance “doggy пойдем” ‘doggy let’s go’ has an English word and a Russian word) and intra-utterance mixing, or the use of different for different utterances in the same conversation (e.g., ”give me some water” when the conversation was mainly in Spanish). When adults use two in a single unit of discourse, their behavior is commonly referred to as “code-switching”, a term that implies that their use of the two languages is deliberate and systematic. We prefer to use the more general term “code-mixing” when talking about children because we do not wish to attempt to differentiate between deliberate and accidental code-mixing, and we do not yet fully understand the linguistic constraints on their mixing.

Source: Language Development in Preschool Bilingual Children
Elena Nicoladis, PhD
Boston University Somerville, Massachusetts