I harped on this topic in our recent ELL workshop and thought some research may be welcomed.
Language, Power and Pedagogy: Bilingual Children in the Crossfire is an eBook by Jim Cummins who has long been a leading scholar in the field of second language acquisition and bilingual education. Here he talks about, among other important topics, "additive bilingualism" (i.e. when kids maintain their home language while learning English) and its importance. He states here that over 150 studies have shown strong evidence for how critical it is to maintain the home language.
Begin reading from page 37, under "The positive effects of additive bilingualism", of Cummins' book Language, Power and Pedagogy: Bilingual Children in the Crossfire:
The rest of the book is worth reading, too! Enjoy!