I have never read anything as to where the children were that night.
I think if the four year old was in the car while NF disposed of Samantha's body, she would have told someone that Daddy took us for a long ride.
JMO
In the original story posts the news reported SH was out at a friend's & NF was home with the kids. Of course, he said he was home with kids... He also shed crocodile tears & blubbered for her to go home.
So nothing confirmed on that point, or whether the grandparents were around.