The research that I know is about foreign language immersion programs, so I assume that is similar to what you are talking about.  I've only read positive stuff about it, nothing negative.

 

In general, the earlier that kids learn a foreign (or second) language, the better.  The research I've read indicates that, in addition to the brain being better able to learn the two languages at an earlier age, there is a host of critical thinking and problem-solving skills acquired by kids who learn different languages at young ages (including the 'language' of music, for example). 

 

US schools have generally not been so great at implementing sound foreign language curricula (It's typically too little too late.)  Some people think that is because the US has been such an economic power in the world, and so Americans don't see a need to speak the languages of other people (because those other people from less powerful countries will learn English--which they do.)  The question is being revisited now, of course, with the recent high numbers of Spanish-speaking immigrants.

 

Linda DiDesidero