Consistency is key in early language learning. If you mix languages in the same conversation, young kids experience difficulty separating vocabulary and grammar into the appropriate language. The child may learn the "mixed" language as one hybrid language.
Parents also need to consider how to strike a balance between the languages. If a child attends school in one language all day and has only a short time to hear the other language at home, it's likely the school language will develop more easily than the home language. Parents have to plan for additional time spent using the home language in a variety of situations and with a variety of speakers. Rich language experiences in both languages are essential for good bilingual development.