There are multiple advantages to learning a second language as a young child. Learning a second language young, it is easier to learn a third, fourth, or even fifth language. The younger a child learns a language the more natural sounding the language is for the child. Some research shows that people who know a second (or more) language tends to be better test takers. Not only that, multi-lingual speakers are necessary for some very specific jobs.
One final thing about learning a second language young is the child's ability to grasp a new language easier. Young children's brains act as sponges. They seek knowledge. Young children do not have to find time to work at learning a new language (given the way they learn is very different than the ways older children and adults learn). This said, they also acquire a greater vocabulary, understand and recognize word origin, and (tend to) have a better appreciation for other cultures.