Thank you for all your comments. I do really appreciate. I specialized in bilingual children and all the issues you mention are REAL questions that all the linguists ask. We solved them out in the following way:
-within the day teachers talk to the kids in their native language ONLY. They provide them individual care so the three of them have no chance to speak to the baby at the same moment in three languages. As you say if teachers speak among them in one language babies will soon try to switch to the same language. And at that moment you have to be very consequent and play "deaf" as the linguist say-you don't react until the child tells you the message in the required language.It works and was proved by scientists. I also experienced that in my life-I am a mom and my child speaks three languages at the age of 8 and switches easily from one to another.
-each day includes 4 hours of activities carried out for all the children in one language ONLY(2 hours in one language in the morning and 2 hours in the other language in the afternoon). While one teacher talks to all the kids in one language other two help the teacher in practical issues like changing diapers, bringing the child back to the class room etc.In the afternoon languages swap.
Day cares in Warsaw vary as you say. I've visited lots of them recently and must say that some of them are fine but there is a portion of "parking lots"-you leave your kid at 9 and pick it up at 5-just like your car. That's pure waste of time, money and especially child's potential. While I was recruiting I also realised that in general potential teachers-especially the ones who worked in the public kindergardens and day-cares don't believe that a child has unlimited capacities and think that as soon as he/she is kept clean everything's fine. It's scary.
We already have a www page so if you are interested in the idea please refer to it
www.dommaluchatrzyjezyki.home.pl
Again thanks a lot for all your comments.