Indy (under Mind Control) slapping Short Round. The boy's reaction is what really sells it. He pulls on Indy's arm, seemingly thinking everything will be okay, only to get struck to the floor. "Doctor Jones...!"
All those innocent children being forced to work in the mines is heartbreaking to watch.
Also the haunted looks of their mothers and grandmothers in the village, who don't even know if their children are still alive.
What happened to the poor shmuck who was sacrificed by the cult just before the good guys got caught.