I think there was never a directive or formal agreement about Grandpa watching the baby. I think as the dad said, little DeOrr was playing, (in my mind I can see him happily pushing his little car in the dirt near the camp fire).
If the parents wanted to leave they might have decided to sort of sneak off quickly like parents sometimes do when a child is sensitive to separation and might cry when they leave.
I think when dad says he saw the baby was "good with grandpa" it indicates to me that it was an opportunity to leave before putting him down for his nap and it sounds like it was not a verbal confirmation with grandpa...just thinking if they left, grandpa would notice and take responsibility for little Deorr's care...perhaps by habit.
All of course, just assumptions on my part as I try to make sense of how this might have happened.
As a retired teacher of young children I have seen many times parents (especially younger ones) just assume someone else is watching their child. In my own experience, I have had parents just walk off and leave their child with me at a school function to go look after another child, etc without ever asking me to watch their child, just assuming I would.
It always would seem so cavalier and casual to me, and even sometimes irresponsible. But frankly, it is not all that unusual.
If Deorr and mother (not sure about father) were living with her other grandfather, they may be used to abdicating responsibility to other relatives informally.
Just today I was watching two very young children running around my large grocery store, just toddlers, chasing each other and picking things up off the shelves. The mother was on another aisle and finally came back to collect them, but within 10 minutes the pair was off running around again..it is just way some people parent. It is all okay until something bad happens.
All of this is just to say nothing sounds to me like the responsibility for watching the Deorr was addressed in a more formal way..dad never said in the interview "I asked grandpa to watch him" or anything like that. He just says DeOrr was "good" with grandpa, meaning to me that all was well, baby was happy and the parents just decided to take the opportunity to go "explore".
I can imagine Grandpa thinks Deorr is with them because perhaps GP was preoccupied, possibly talking to his friend, and was not aware they ever intended him to watch him or perhaps he just noticed him toddling off and assumed he was joining mommy and daddy.
Casual parenting style, lack of communication, no one assuming responsibility for watching him, a fast moving stream, and an active mobile toddler all leads me to still believe falling into the creek and being rushed away in the water is a logical assumption at this point. JMO of course.