As you mention this,
@Mushroom, I must tell you about a documentary I watched about Natalie Wood. When she was a very young girl, she was in a scene where a bridge over a river was meant to collapse just after she crossed it. But it collapsed too soon, and she nearly drowned in the river, clinging to the bank and struggling to escape. Unbelievably, instead of rescuing her, they let the cameras roll as it made good footage! She had an injured wrist for the rest of her life.
I'm sure film-making ethics have improved today. I think a lot of green-screen technology is used for filming children. You make a good point about their lines, though.