Research has proven that children learn languages more easily in the early years. Since their brains are so feverishly making new neural connections. Studies have shown that kids can hear unique sounds of a foreign tongue better than an adult or a teenager.The best way for a child to learn a second language is not through audio but through conversation, with a live person, the child observes and repeats easier and it helps the brain remember faster. Also, the child needs to hear the language as he/she grows, to continuously learn. Having a friends or babysitters who is bilingual would be a help. Have them speak their native language to your child.
When the child grows up in a bilingual family they have a lasting cognitive benefits. However, their vocabulary is less in each language, but combined languages they know as much words as a person knowing just one language.