The weird thing here, to me, is that apparently all four bodies were found together (I think). That would confuse the medical examiner. You'd think that one would go down (but then, another one would then administer first aid). Surely they were trying to cool the baby. I hope, I hope. If they suddenly discovered that the baby was unconscious and not just sleeping, as they had thought, then what would they do? Would they realize it was heat? I think so. So what then? Their water container would have been out of the pack or off their back, right?
Wouldn't one of them attempt to run the baby to the safety of the air-conditioned car? If the baby was already very ill, perhaps they stopped to administer whatever first aid they could, and each one rapidly. passed out in succession (sorry for the grim speculation, but usually the ME would expect to find heat stroke victims spaced out - as each adult would continue trying to move toward shade/safety, taking the baby...)
MOO
I have a tentative theory, which I’ll abandon in an instant if any more evidence shows up.
Let’s suppose that Jonathon, carrying the baby, feels the effect of heat and exertion the most. They decide that he’ll sit down, rest and recover, and then they’ll go on. Naively, they don’t realize that he has heat stroke, and that just resting and drinking water isn’t enough.
The dog is on a leash, fastened to Jonathon somehow. (We have not heard for certain that the dog was on a leash, although one report implied it. There are certainly good reasons for having your dog on a leash—like, what if you do meet up with a rattlesnake?)
So, Ellen stays with her family, and becomes more and more overcome by the heat. (Even when they first stop to rest, she might well not be thinking as clearly as she normally would, again, from the heat.) Finally, she realizes that the only chance for their survival is for her to get help. She might not realize that Jonathon is getting worse, rather than better, until he becomes unconscious. (As for her getting help, I don’t think we’ve heard anything about where the car keys were, or where her phone was. Maybe she left it in the car, even.)
So, why she doesn’t take her baby? Maybe, in a moment of coherence, she realizes that with the added weight she would have no chance of getting help—and the baby’s survival depends on her getting help.
So, she starts off to get help. The added exertion of struggling up that hill pushes her over the edge. She doesn’t get far. The baby and dog, completely helpless, both die.
This is entirely my speculation. I think it’s consistent with what we do know, and it avoids the question of: “how could they all die at once?” The answer would be: they didn’t.
MOO