Mixing languages is usually a sign of high competence in both languages. Bilinguals often do it even within a sentence (known as code-switching). This does not mean they cannot speak in just one language, or that they are not sufficiently fluent in both languages. It simply means that these speakers use their whole potential in communication, i.e. using the expression that best fits what they want to say. Some studies have found that speakers who are very fluent in both languages are the ones that mix the most.
Does raising your kids to speak two different languages confuse them? Read on at 2 Langauge 2 Worlds to find out in this study done on 606 five-year old children. The impetus of the study is to look at code-mixing in children with and without language impairment.