Perhaps the grand father is the original soot user, possible a human with the ability to create soot. That soot carries maybe some psychic energy with it that tries to imitate humans. Bring in the kids, the shadow's take the likeness of the children they're with, and ultimately they merge...
@tsuu that's still an adult inside, capable of eloquent writing and explanations of the situation and thought process. Even if the memory would fade, you'd give some serious consideration later, having read something a young child would not be able to pen down.