Wait - maybe I missed something, but why couldn't they have been killed before 5:08? They had been out walking since somewhere between 4:30 and 5:00, right? For all we know, they could have been killed on their way TO the bridge. Maybe?
Since we don't have much to go on in respect to a time line, I think we have to consider what we do know and go from there to reconstruct what probably happened.
It was a hot Sunday afternoon. Sometime between 4:30pm and 5:00pm the two girls decided to go for a walk. Grandma was probably in the kitchen beginning to plan/prepare the evening meal.
At 5:08pm Skyla's mother calls to tell the Plackers that she's coming to pick up Skyla. Grandma and Skyla's mother casually chit-chat for a couple of minutes. After she hangs up, Grandma calls Taylor's cell phone to tell her that Skyla's mother is on her way to pick up Skyla, and to hurry up and come home. There's no answer. So, she gives it a couple minutes and calls again. Again, no answer. At this point, no one is alarmed, probably thinking that Taylor's phone is turned off or that the girls aren't paying attention.
Grandma tells Grandpa that Skyla's mother is on her way and she's called Taylor's cell phone and there's no answer. Grandpa decides to walk down the driveway to the road and see if he can spot the girls and shout to them to come home. He gets down to the road and doesn't see the girls, so he starts walking down the road. At this point, there's still no cause for alarm, or a rush to find them, as it's going to take Skyla's mother 15 - 20 minutes to get there.
We know that when Grandpa walks down the road, he discovers the bodies of Taylor and Skyla. He, of course, goes into shock at what he sees. For a few minutes, he was probably too stunned to react.
I don't think there was anything that afternoon that caused the Plackers to be alarmed, until the bodies were discovered.
BUT.................with Skyla's mother calling at 5:08pm, and considering they talked for 1 - 2 minutes, I would put Grandma's first call to Taylor's cell phone at about 5:10pm. With no answer, I think the girls were already dead by that time.
This puts us into a 30 - 40 minute time frame. If the girls left the Placker home about 4:30pm or shortly after, and they were dead by 5:10pm, whatever happened, occurred within that 30 - 40 minute time frame.