Minor point: the Grandchildren were visiting the parents.
The parents were not in the house when the fire occured. . .
I think you mean, grandchildren were visiting grandparents.
And apparently, it was overnight.
Not a shock that no one would be either aware, or up and ready for a fire, especially if it started in the far reaches where no one could notice until too late.
Who knows, maybe it went boom (gas), but right where they were. A boom would be a good alarm, but not if in the wrong place!