It's rare, possibly unprecedented, and very chilling, to hear a recording of someone speaking to his victims just before he assaults and murders them.
Perhaps because we hear fictional, imaginary, versions of this every day, it seems insignificant.
But I believe, in this context of a ruthless killer taking control of two young girls that he intends to exterminate, the word he used to address them is significant.
I think a less friendly approach would be to say "you"or you two". I also think it would come across as more menacing for a man to say 'girls' or 'young ladies'.
I think he was using language that he thought would express authority but not terrify them.