I think it is difficult to do studies on this matter, for the very simple fact that people who are bilingual tend to be more worldly, and people of this sort tend to be more intelligent. So, it is not unreasonable to assume that their children will be just like them, bilingual and intelligent.
All I can say, as a linguistic apprentice, is that it is important to teach these things young and while studies showing correlation between increased lingualism and increased intelligence may be troubled, there are no studies at all that have shown knowing more languages makes a kid dumber.
Kids' brains are very maliable, and they can straighten out the input into the right language with the greatest of ease.
Jon
P.S. Imperial system”and its derivatives”rules
You've been Gremled!