This may not be it, but there are enough similar elements I thought I'd give it a try:
"Dear Devil" by Eric Frank Russell. Mankind nearly wiped themselves out a couple of generations previously with some sort of plague-war. The details are fuzzy because the only survivors were children who were abandoned by their parents so that they wouldn't catch the disease. After a couple of generations, confused recollections more or less boil down to disease, terror, fire, and everybody dead. There are very, very few humans left.
Into this comes a Martian. He lands and finds a small group of children, convinces them to trust him and take him to meet the one old man they knew. He was either a survivor from the war or the son of one and barely remembers what happened, but the Martian is able to puzzle together the story.
The Martian works with the children and helps them gradually develop from a loosely organized tribe scratching out existence into a true civilization, relearning the skills and knowledge of their ancestors and rebuilding civilization.
In the end, much time has passed and the Martian is dying. All of human civilization is mourning for the passing of their savior, and the last of those who had once been children comes to visit him and bid him goodbye and to finally get him to admit why he came to Earth in the first place.
The title comes from when the humans first meet the Martian and think that he matches the description of the Devil. But since all the religious folks pretty much died and they grew up a few generations removed from religion, it was more a taxinomical term for them rather than a figure of fright.