I have read somewhere in the comments on one of the chapters that the shadow people are maybe the shadows of the living dolls, and I strongly agree with it.
So here's my theory: I think the grandfather is some fantasy creature who steals children away and erases their memory. Then, he uses the human's shadows and create the so called "Shadow people". The coffee in this chapter might be a substance that the grandfather created that prohibits living dolls/shadow people to find out what he really does. Think about it: We've never seen the shadow kid's parents, (or rather very little of them, to the point that they're non-existent), nor have we seen if they have any other relatives such as cousins, uncles etc. The only exception were the twins we saw in the first chapter, but they could've also been stolen altogether.
All in all, the plot thickens and I'm anxious to see what's going to happen next.