There's no causal relationship between a third party having a dream and another person dying. Not unless they wake up from a dream and go and kill the victim, at least.
In so far as trying to explain why this has happened, it may simply be a coincidence - just because it's unlikely doens't mean it's impossible - of perhaps something was said to your daughters which caused a certain pattern of thought to develop and this was revealed in the dream. As for your grandson, you don't say how old he is, but perhaps his parent mentioned it in front of him and he's simply repeating something for attention, or it similarly set up a line of thought with him to.
Anyway, bottom line, they can dream it all they like but it doesn't mean anything could conceivably happen to you as a result of it.