It’s no problem if you want only a crying or non-crying baby. (Well, sometimes it is, as I’ll mention later.) What I mean is, just one baby for just one scenario, whatever the scenario.
If, for instance, you want a baby in some kind of ad, you put out a casting call and parents bring all sorts of babies who fit the criteria to the site (in the cases I’m familiar with, the ad agency’s office). The director/assistant director comes into the room, fixates on a baby who has the correct mood at that moment, grabs said baby and its accompanying parent and off to the studio they go.
Crying babies are actually a problem, in that a baby who’s crying at the moment of the (for lack of a better word) audition may well get all distracted by the cameras and lights and props, and be curious, and cease crying. The setup is perfect if the director desires a red-eyed, tear-stained, but not actively crying baby.
Conversely, the happy gurgling baby might get intimidated or frightened by the cameras, lights, etc., and start bawling its head off. In which case it just flunked its tryout and the director returns to the casting area and tries another one who’s exhibiting a proper mood.
Now if this all sounds pretty heartless, think of the receptionist out there in the lobby with all those stage parents and babies. Note that a baby who stars in a commercial will get paid more for a few minutes’ work than the receptionist does in a week.