Q. Will mixing languages in our bilingual household confuse our child and even delay his speech?
A. My practice is full of bilingual families, most of them Spanish-speaking, and my experience has always been that children who grow up hearing and speaking more than one language do just fine. Sometimes, though they clearly understand both languages, they will wait a little while to say their first words or have periods of time in which they don't talk much at all (most likely as they sort out the words). Mixing the two languages is common as well. But research has shown -- and the kids in my practice bear this out -- that being bilingual doesn't slow language development.
In fact, being bilingual has its benefits, which is why I encourage these families to use both languages with their children. Preserving family heritage and culture is a great gift, and knowing another language can open up the world for children (I've become fluent in Spanish, and it has truly opened up the world for me!). There's also interesting research that shows that when kids are aware there are different ways to say the same thing, they're better able to grasp abstract ideas and symbols as they grow older.
Which is not to say that if your family speaks just one language you need to run out and sign your child up for Berlitz. There are lots of other ways to encourage his intellectual development -- like reading with him, drawing and making things together, playing imaginary games, or visiting places like the zoo or a museum.