What we do: Very strict OPOL During the week the child is to 75% exposed to the German language. I talk German with him all day, we have German TV, he watches only a few programs in English. We have lots of German friends, go to two German playgroups and only have a few play dates with English speaking children. We have lots German books etc.
However, he is two and does not talk at all. He just had surgery for his glue ears and we hope his hearing improves now.
Watching with interest as my dd (23) months is getting a wee bit confused. Me (English), DH (German), and nanny (south East Asian) all do OPOL.
DD now will create sentences like Go (Asian language) Book (German) Please (English). Am sure it will all come out in the wash but she can't be understood by anyone who does not speak those three languages...
Ophelia My DS (2.5yrs) does that too - he mainly speaks English, but every now and then, he'll use a German word for specific things. I reckon that they will be able to distinguish and switch between languages when asked to when they are older.