Was it the mother's fault the boy was so soft?
It seemed the father was a pragmatist from the start, and the boy being so sheltered and naive was at odds with that.
Is it possible the mother pitched a fit every time the father suggested he get the boy outside and/or taught him some survival skills?
The way she surrendered herself to death in the end might suggest she had been so totally unwilling to face the reality of survival (understandable in such a nightmare world) that she'd simply refused to let her child be exposed to it either.