Warm, engaging and a lovely premise about an awful topic. It is impossible not to fall in love with such a wise child as our six-year-old narrator here.
Left as an "only child" who used to have a brother, Zach navigates grief in his own way, since his parents are doing their own grieving in their own way. Zach is confused and almost forgotten while his parents deal with the death of their other son. So he figures out a way to cope on his own. In the end he saves himself.
I don't know if any child would actually have as much insight as Zach, but he should hang a shingle and do couples therapy.