That's a bit like asking "what do you do with a child who, at 7, is given access to the nuke button and chooses to press it in total ignorance".
I don't know, what do you do with a child victim of such a ludicrous situation?
Obviously the grandfather need never see the light of day again for ending one life and ruining another, but as for the child? How does anyone ever get beyond that? Many would double down, but really, a lifelong commitment to psychological treatment? Let's be real, they can't ever live a normal life, so I guess that's the best you can do. Tell them none of it was their fault, try to make amends, it's not going to be easy.