Early language learning, the brain and the mind.
Language learning enthusiasts have always known that children, particularly those under the age of six, learn language more rapidly than adults. Children have an amazing ability to remember and reproduce the phonemes ( basic sound units ) of any language. It might seem like we are putting a lot on their little shoulders, but so long as it is fun and the children are not being forced into doing something that they do not want to do, I do not see the problem. You will not hinder their language development in any way by exposing them to foreign languages.
Whether or not you should expose your children to two or more languages is entirely a personal matter. Personally, I think it is a sign of good parenting to regularly expose your children to two or more languages. But that is just me. Some parents, particularly anglo-centric Americans who can only speak English are down on language learning. ( I think they are jealous of us :-)