I have read stories of people surviving black widow spider bites in real life. I even read of a 5-year-old girl who survived one. Alexander must not be very tough.

Actually, the entire family must not be very tough. Rosella and Valanice can die from spider bites in "King's Quest 7," and there are kids in the game who climb the spider's web without problem. And Graham gets killed by a spider if you take too long to escape the witch's dark forest, or if you don't have everything you need to escape the dark forest (i.e. the honeycomb).

The spiders in the world of King's Quest tend to be larger than those we have in real life, and therefore surely pack a more lethal dose of venom.

For example, the spider of which you speak from KQ5 is rather large, being roughly the size of Graham's torso.

The black widow from KQ6, as I recall, is larger than Alexander's hand. In reality, black widows are less than two inches long.

The kids in KQ7 are already dead, aren't they? What do they care of being bitten?