You spent much time with Kids?
You’re gonna need atleast triplets for that kind reliability. 
They send an Assistant Producer down to the hospital, where they have a room with a big glass window, called a “nursery”.
Then they grab one that looks good, leave $5 to cover the inconvenience, & when they’re done…they just throw it away.
In other words, the same way Hollywood treats most people.
The stork room?
The scenes I can’t wrap my head around are the ones where the baby/toddler are crying.
Who could stand there watching their baby cry for no good reason and to make money?
I was watching Steel Magnolias on TV (guilty pleasure…) and there is this scene where the 1-year old is screaming his head off, pointing and reaching (probably for his off stage mommy). It kills me.
Y’know, I read muldoonthief’s post entirely wrong. Nevermind…
How to make a baby in an oven (2minute video, SFW, has leading ads). These are using in films and TV, as well as to give comfort in nursing homes and so on.
I find it interesting that in a show like Lost, where Turniphead/Aaron has to be a baby of 0-3 months age, but over a period of three years shooting time, meant that he would’ve been played by probably as many as 20 babies, of either gender.