Good news for parents worried about raising bilingual kids. New research from the University of British Columbia finds that babies begin to distinguish between languages with very different grammar structures beginning at about 7 months.
Young children use word duration, frequency and pitch to understand how words work and sentences are built. For example, when a child hears English, he understands that a content word, like dog, lasts longer and has a different pitch than the preceding function word, like "the". But when a baby hears language where the reverse is true, like in Hindi or Japanese, he registers that as well. This is especially important since previous research has shown that monolingual children use word frequency to understand how important words are.
So if you're worried about confusing your children with two languages, don't worry; they'll get the hang of it.