I don’t want to hijack this thread, but adults and children learn differently – a child isn’t necessarily a better language learner, when speaking in terms of speaking fluency, versus academic language. What whiteowl and aisyna said are most important, but what really is most effective is language interaction, which is why audio-lingual and rote memorization techniques ultimately are ineffective. Make the language COME ALIVE by not only having your child hear it, read it, and interact on a computer, but have them USE IT and ENCOUNTER it in meaningful areas – or through the child’s favorite topic. For example, if your child is crazy about mummies, bring him/her to a museum and talk about the mummies ASK QUESTIONS about the mummy in the target language. Give the child an exciting and stimulating reason to PRODUCE the language with you or other children. Arrange play groups in that language. Play word games while you drive – because language is really another set of synonyms that they child can encounter.
See:
http://www.cal.org/resources/Digest/digest_pdfs/RaiseBilingChildi.pdf
http://www.multilingualchildren.org/
http://iteslj.org/Articles/Rosenberg-Bilingual.html
Hope this helps! They are very interesting articles, especially the first and the third.