Having lived in France since they were 4 and 5 years old, our boys are both French-English bilingual and speak French far better than we could ever hope to. Hopefully this will give them a huge advantage when looking for work but there are some other perhaps less well documented pros and cons for both them and their parents.
Advantages of being bilingual for children:
They can slip into French so their English speaking friends cannot understand them and vice versa. They can try and swear in French and hope I won't realise ... although I am becoming wise to many of the "gros-mots" they attempt to slip past me! They can speak and understand jokes in 2 languages.