You see babies in movies and TV shows, but they aren't really actors. They don't take acting classes or join a union. But they are considered employees, and the legal bodies that oversee movie productions have specific rules about infants on the set, or at least they do in some states. It's pretty complicated.
To get babies onto the silver screen, Hollywood came up with a lot of workarounds to existing child labor laws. Some are more ethical than others. Let's just say that if you give birth to premature triplets, there's a way of making them earn their keep. -via Tastefully Offensive