In the beginning, when we heard BM's 911 call, I assumed she had both children with her as she walked out to the mailbox. It seemed that that she was speaking to someone else, that she was out of breath as if she was caring a baby and I remember her explaining we're going to the mailbox (which sounds like something you would say to a three-year-old.) At first I thought BM said at the end of the call, "please don't hurt me." That didn't seem strange because a six-month-old could be pulling on your hair, struggling to get down or pinching your neck. I was very surprised, very, to learn she didn't visit Frankie's trailer until after she called 911.
ETA: I would have assumed she would find a similar scene of slaughter at the other trailer since there was no sign of life there and the trailers were so close together surely they would have heard what was happening and done something to help if the could.