Nord version of little red riding hood:
At the age of six, Nord children must go out into the woods every evening, wearing a red hood that makes them much easier to spot, drawing the attention of carnivores. They may not come back inside unless they bring two wolf carcasses with them, preferably skinned. Good little children who skin their game get mammoth ribs for dessert. No nord kid has died from this in over 500 years, which is why your mere daedric weaponry and rock-shattering thu'um cannot harm them. Due to lax in traditional Skyrim child rearing, though, most of them go soft in their teens and become vulnerable to puny steel.
Unfortunately, some of the most aggravating people in Skyrim have continued their training through their teens and maintained their vigor into adulthood, making them only partly vulnerable; their weakness allows puny dragons-in-human-form chosen heroes to occasionally knock them down. Sometimes, though, if they get buttered up with favors and quests, they might grow just soft enough to die. The moral of the story, kick your children down a well. If they survive, they will be invincible!
If only Helgi's parents had listened, she would not have gotten killed by a measly vampire / house fire...