Dh just went and bought War Horse and another similar book (The Elephant in the Garden) for ds who is six. I think this is FAR too young to be reading about such dark stuff.
Ds is quite laid back and doesn't seem to have nightmares but this might be pushing it.
There are so many nicer books than these for a six year old, even if he is smart he doesn't have the emotional maturity of a ten year old or whatever.
I'd much rather he read something like Tom's Midnight Garden. AIBU!?