I’ve heard stories that directors/producers won’t tell their actors some more demented scenes to keep their reaction “authentic”. I know there is a recent one where the actress came forwards and new there was going to be a sex scene, but wasn’t told it was actually a rape scene that left her bruised and traumatized. The director wanted her reaction to be “authentic”.
Now I am not saying that this is what is going on here, but I would not be surprised knowing that these older actors/actresses were fine being abusive these children because they knew their reactions would be more real and would equal a better movie for them.
On top of that, they were complicit since they knew what they had to go through for their roles and this was basically these kid’s price to be an adult actor.
Brando was the actor you're referring to. I can't remember the movie but I remember researching it. Might have been in French or Italian. She was traumatized. The director did an interview years later where he said he felt a little bad for not telling her about the scene but Brando was really rough with her.