What does the helicopter say to reassure Lenina and Bernard that they will be safe during their stay at the reservation?

I think this might be the quote you are looking for,

"And with that he climbed into the plane and started up the engines. "Back to-morrow. And remember," he added reassuringly to Lenina, "they're perfectly tame; savages won't do you any harm. They've got enough experience of gas bombs to know that they mustn't play any tricks." Still laughing, he threw the helicopter screws into gear, accelerated, and was gone." (Chapter 6)