Such evolutionary just-so stories can be used to explain anything and there is no way to falsify them. However, just applying common sense: Wouldn't it be more beneficial for their survival if kids were programmed to stay away from megafauna? It might eat them or step on them.
How do you know how to stay away from something if you don’t understand it? It’s big enough that it can come to you in the middle of you doing something else. Best to know what habits of its might make that happen.
That, and the megafauna that have existed alongside humans have pretty much always been herbivorous mammals. Mammals seem to be universally capable of recognizing which members of another mammal species are children of that species, and are less wary around them, and tend to (for the herbivores) even be intentionally gentle toward them. Mothers of many mammalian species even choose to adopt and care for orphaned children of other mammalian species that don’t look particularly like their own species (and I’ve never heard of this happening with a non-human mammal mother adopting a non-mammal, or a non-mammal adopting anything, so AFAIK it seems to be mostly a “mammal thing.“)
Of course, this same awareness of child-ness leads the carnivorous mammals to think of another species’ children as easy prey. I think humans do have an instinctual understanding of the difference between a herbivorous/“friendly”-looking species, and a carnivorous/“predator”-looking species, and human children do seem to have much less of a desire to attempt to wander up to lions or Tasmanian devils than they do to approach e.g. elephants or giraffes (or horses or cows, for that matter.) And even the kids that love dinosaurs certainly have very little compulsion to approach crocodiles, snakes, or birds of prey. They might think they’re cool—but there’s a component of fear to that admiration.
Humans don’t seem to have any instincts to avoid approaching omnivorous megafauna, and that’s seemingly a hit-or-miss proposition, but maybe this is where the perception-of-child-ness thing comes most into play. An adult human definitely shouldn’t provoke a gorilla or a bear, because an adult human is a viable threat to them, one they feel they have to respond to; but a human child won’t be as easily read as a threat, and might just be ignored, the same way humans don’t really worry about gorilla infants or bear cubs getting near us. (That, and there just aren’t that many omnivorous megafauna, so maybe we don’t have any instincts about them.)
So kids can instinctively discern between a large herbivore and a large carnivore, but cannot instinctively discern between a large herbivores and a garbage truck?
