CAL, the Center for Applied Linguistics, has published an online document, "Raising Bilingual Children: Commen Parental Concerns and Current Research," that describes a research study by Kendall King and Lyn Fogle that looked at 24 Spanish-English bilingual children ages 0-5. The conclusions are as follows:
• Although many parents believe that bilingualism results in language delay, research suggests that monolingual and bilingual children meet major language developmental milestones at similar times.
• Despite many parents' fear that using two languages will result in confusion for their children, there is no research evidence to support this. On the contrary, use of two languages in the same conversation has been found to be a sign of mastery of both languages.
• Many parents rely heavily on television to teach the second language; yet this is best considered a fun source of secondary support for language learning. Human interaction is the best method for fostering language learning.
• Contrary to the widespread notion among parents that bilingualism results in "bigger, better brains," parents more realistically can expect their bilingual children to gain specific advantages in targeted areas, such as greater understanding of language as an abstract system.
I really recommend this article--it is brief enough to read in one sitting and serves as a good introduction to issues and ideas relevant to raising children bilingually. It also offers a lengthy bibliography to serve as a springboard for further reading.
I'm curious to find out from parents of bilingual children if this study resonates with you--did you have, for example, the same expectations that learning a second language would make your child "smarter"? And do your observations of your children in general match those of King and Fogle as documented in this article?