My daughter is 4.5 now and she is fully bilingual (English/Russian). Russian is a first language for both me and my husband. And we have a rule at home we speak strictly Russian unless we have guests who don't. We tried to minimize her exposure to English until about 2 yrs old. by that time she was very well spoken in Russian. Then she went to the English speaking daycare and peaked up English I a few month. She right away understood the concept of different languages and never mixed them since(mixing is very common for bilingual kids). Understanding of the concept
seems to be the most difficult part for many little children.
It is hard to them to understand that to different names of the same object belong to to different groups of words called language. As soon as they learn it it becomes much easier.