What to do when bilingual kids mix languages (code-switching, a.k.a. code-mixing)
• Don’t worry if your child mixes languages—language mixing is a common (and typically short-lived) phase of bilingual development.
• Trust your child is not confused—she may not know (or be able to explain) that she’s using two languages, but there’s plenty of evidence to suggest that she has two linguistic systems.
• Understand a bit about how and why children mix when evaluating your child’s language use.
• Minority languages may need extra support, and frequent use of both languages together can make it difficult to keep an eye on the support for each language.
• Set realistic expectations for your young learner—there are no perfect bilinguals in the world, and remember that language learning is a lifelong process—it’s never done.
Source: King, K., & Mackey, A. (2007). The bilingual edge: Why, when, and how to teach your child a second language. New York: Collins  (p.184)
How to motivate children to learn a 2nd language (p.267)

Online lessons for kids: dinolingo.com