There can occasionally be small spots or even patches of skin without any melanin. Any hair that grows on those spots won't have any color. I have several tiny spots (not much larger than 1mm) on my arms, and I can only see them when I have a tan. But any hairs growing out of them are white. It's possible your daughter has the same thing, but on her head. My brother has always had a 1" spot on his head that has no color, and the hair is white, so luckily she doesn't have that much!
It's harmless and natural, just "one of those things"! I believe it's genetic, and have not ever been able to find a name for it. So your husband & father are right - no need to worry!