I've always been very relieved an grateful that DS6 is so ridiculously logical. He rarely has any of the sort of irrational but very scary fears that HG+ kids usually have.
His biggest fear hit him when he was 3. The preschool guinea pig and his great-grandfather died within a month of one another, and the 1-2 punch was really devistating and terrifying for him. I think that because fearing death is not illogical and can't be waved away easily, it was all the more difficult for me to deal with. People really do die. He will one day really die. It's a valid fear.
I went with the standard, "everything that lives will die eventually, but you and Mommy and Daddy will all live a long, long time." It wasn't enough. He chewed on death for months, acting it out in his make-believe play, talking about it incessantly, and even having trouble going to sleep (which he never has trouble doing!). Hugs and reassurances, discussions of the science of death, talking about religious beliefs...nothing helped.
Eventually it just played itself out. I think he did what we all do and he decided not to let the fear of death keep him from living. But I can vividly recall how worried I was for him, how powerless I felt to help him.
Their fears become our worries, don't they?