I don't see any mention in the story about the kids actually being threatened by the snake. The way I read it, the family comes out to find a dead snake and a snakebit dog.

I'm really happy the story has a happy ending and all. But I'm with Duane--I think the parts about the dog 1.) being able to distinguish a poisonous from a harmless snake, 2.) foreseeing a potential risk that might happen if his people encountered it, and 3.) acting on that expertise and forethought...are not reasonable assumptions. IMO

True. Personally I think it is within the relm of possibilty to believe that a dog can sense a dangerous animal through scent or body language. But yeah, besides that I'd agree that the rest of their reaction is just instinctive prey or fight drive without any foresight.

A dog can sense dangerous "intent" in a bull before I can. But a snake, I don't know. Do snakes have intent? I'm not sure.

I've never owned a terrier that didn't try and kill any and all snakes they encountered. I never though of them as heroic, just nucking futs!
Snakes are purely reactive when it comes to biting in defense. The poisonous ones can't even control their venom in a defensive bite. You have a 1/3 chance of a "normal" load of venom., a 1/3 chance of an over load, and a 1/3 chance of no venom injected at all (dry bite).
I suppose you could call it "intent" when they hunt but like us, when the belly is empty we eat.
In a defensive situation I would guess that the snake's adrenaline is in overload. That could "possibly" trigger the dog to perceive that as dangerous intent but I suspect it's more about prey on the dog's part.
Having done some herding I think the dog is reading body language. Maybe reading something in the eyes of the stock, the flick of a tail, a dropped head,ear twitch,etc.
Ive seen herding trials where cattle or sheep may take off simply when the dog walks in the pen or field while the same stock may immediately challenge a dog they view as weak.
One of my own GSDs had no problems with a goat that was known to punish dogs. He simply walked up to it with "intent" and the goat quickly backed off. He did the same with a group of cattle that wouldn't move. He slowly walked up and just about touched nosed with the closest animal and they simply turned and moved off.