
Rex R. answered 04/25/19
MFA in Film with 10+ Years' Professional Experience
Crying on cue is a basic tool that any serious actor carries in his or her repertoire. It can absolutely be learned with training and practice, with different acting techniques championing different processes for achieving real tears. That said, this anecdote speaks to your question about young actors tasked with crying scenes:
There is a scene in the Oscar-winning film Kramer vs. Kramer where an 8-year-old cries after hurting himself on a playground. Before the scene started, Dustin Hoffman, who plays the boy's father, said to him, "Hey, we're almost done shooting this movie, isn't that sad? All these nice crew people, and you and me... we're never going to see each other again." The boy began to cry, and they started the scene.