Is the relative elderly? The father looks quite young so I wouldn't have thought so. How do we know how agile this man is?
My dc are young adults now and I don't know anyone who hasn't lost sight of their children for a short time, or perhaps a longer time.
One of my own dc was lost for close to an hour on one occasion. Fortunately, I did not know about this until after the event when he had been found. He was with 2 close relatives and we are a nice, normal family who love our children.
We all know that we really cannot take our eyes off young children for a second but we are also human beings. I doubt anyone would have given this situation (a grandfather taking his child to the local park) a second thought had this not happened.
Also, I believe the young boys grandfather had fallen over and could not reach the boy quickly enough.
I doubt very much that you would tell a friend they had been 'neglectful' under these circumstances. But, you are behind a screen, don't know these people and your words are insensitive and the fact that you have written them is cowardly.
I pray they find little Noah alive and well today.