When she heard Kaylee go upstairs (she's really hearing Xana here), the dog started barking around the time she said "someone's here." The dog continued barking in the house, but then the sound got more distant, which told her Murphy had gone outside.
Earlier in the night it appears that Dylan saw Kaylee and Murphy go upstairs, and it seems that it was at this point she heard them playing. So my guess is that Dylan went to bed at this point, and the playing she heard was well before the murders.
She falls asleep, and thinks this is the same event. But it's not.
Kaylee and the dog are playing. They go to bed. The murders begin. Xana says "someone's here", and the dog starts barking and the final murders occur.