I was wondering, where do they get such young children when needed. A baby, well a doll, some strategically done wrapping and camera angles can suffice, but with say, a child of a few months age?
I would imagine they would be the children of the actors or of one of the crew. Easier to keep the kid calm if mother is around.
There are talent agencies that specialize in young children. I was once part of one. As for the really young babies, I’m not sure but I would suppose the parents would have to get the kid involved first (I suppose the mother would want to be perusing talent agencies during pregnancy, which is a bit creepy). Child labor laws usually require the parents on set, so the mother would always be around anyway (which wouldn’t make it a particular advantage to use the crew’s kids).
My brother works in film, and last year was asking around if any actor friends had babies for a commercial he was working on. It was much more convenient if one of the parents was an actor since they were part of the actors union, were aware of all of the rules and regulations, and know how to behave on set. Agents would know if their clients had newborns and might get a call to ask if they were interested.
So a regular agent might be a possibility.
^
I would imagine and considering the production times involved, I am sure that it would be so much more convinient tp do so.
The various "making of"documentaries I have seen also indicate that babies/ v young are children of crewmembers etc. Was a bit curious.
They like to use twins so they can double the time they can film a scene with the kid. Ever heard of the Olsen twins? 
The baby at the end of Star Wars episode III was a child of a crew member.
Many parents are very interested in getting their kids into modeling and figure it’s never too early to start padding that resume. Someone I know (through an indie film short my wife produced as a volunteer) has been saving up money for her daughter’s college fund by putting the daughter in the odd movie or photo shoot, starting from infancy and she’s now six or so. There’s so much networking that goes on among film and advertising people that she’s never needed an agent, and she doesn’t pursue it like some kind of profession. A hundred bucks here and there adds up quickly when you can invest it for 18 years before you need it.
Brüno has a really interesting scene where he is auditioning kids for a photo shoot, and he’s asking them purposely ridiculous question about what the parents will let their kids do, and they are all so eager to get their kids hired that they will agree to anything.