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Here is a theory that would make the most sense.
Grandfather decides to move the Suburban or camper trailer a bit .... re-position it in a better spot around the campfire because they were expecting additional family to arrive later with their own tents or campers .
He was aware the the boy was around , but when he checked his mirrors and glanced outside the boy was no longer there , assumed he had gone down to the parents , and did not realize he had driven over , or parked on top of him.
The parents DID go exploring like they said ..... Grandfather DID know the boy was around the campsite ..... Grandfather DID look away for a few seconds and the child was gone ...... and when the parents came back the boy WAS MISSING like they said ... and Grandfather WAS genuinely shocked to hear the boy was not with them.
And the three of them DID do a frantic search and could not find the boy until they regrouped at the campsite and spotted him underneath the vehicle or the camper trailer.
That would also explain the blood or fluid residue the cadaver dogs alerted to on "equipment"
I think they are being truthful about all those things but they become untruthful after that and decided to cover it up for some reason. Issac is off fishing and unaware of what happened. Grandfather can rightly claim he did not do anything (he did not realize he did it) ... parents can rightly claim they did not do it ... there were no witnesses , and there was no crime until the cover-up began
(From the PI article)
“We believe it is important for the public to know that the cadaver dog did hit in three certain areas of the pieces of equipment which does indicate there was a death or there was some type of fluid, cerebral fluid, blood whatever it may be, that was in the location where the dog hit, and that we know there was a death,”
http://m.localnews8.com/news/private-investigator-cadaver-dog-proves-there-was-a-death/38307992