It's pretty shocking to think that not so long ago (for most of the 20th century, actually), "experts" believed that bilingualism interfered with and inhibited a child's intellectual and academic development. They were so wrong, though.
In plain (monolingual) English, that means being bilingual makes you smarter—but that's not all. Now you can add developing "better social skills" to the mounting benefits of raising bilingual children. A recent study conducted at the University of Chicago found that bilingual children were better than monolingual children at communicating because it was easier for them "to consider someone else's perspective to understand their meaning."
This ability seems to stem from the fact that multilingual children often find themselves in situations where they have to consider the perspective of others based on things like "who speaks what language to whom, who understands what content and the times and places in which different languages are spoken."
And there's bonus good news! This study also found that children who only speak one language but are raised in an environment that exposes them to another language regularly (emphasis on regularly; think grandparents who speak a different language in front of the child) were also better at communicating and being able to interpret things from someone else's perspective than children who are not exposed to multiple languages. Maybe that's what studies have also shown that bilingual kids earn more money as adults.
So, while exposing your children regularly to another language might not raise their cognitive abilities (i.e., IQ) the way that raising bilingual children does, it can still give them a social advantage by helping them see the world and those in it from a different perspective.