Except from what I understand the 4 year old sister cooberates the grandmas story, but yes, I agree, no clues no scent for the dogs, nothing.
If the boy was present at the scene because the 4-year old girl corroborates grandma’s story, it’s not that tough to fool a 4-year old! Boy already dead grandma says ‘he’s sleeping’, etc.. Then she hides boy’s dead body back at the car and tells girl ‘he’s missing’, etc.. ELABORATE HOAX BY GRANDMA!