Is it possible that some sort of chemical fumes got into the room? Enough to make the babies really sick, but not so much that it would kill adult rats? Has there been an exterminator in the house? Or something unusual got burned nearby? Anything at all sprayed near those cages? Or any other source you could think of for something toxic getting into the air? Or maybe ambient temperatures dropped too low and the babies didn't fare as well as the adults? I'm just thinking that from everything I've read, it sounds like some sort of environmental problem.