This moment is difficult for the narrator because he doesn't believe his daughter is old enough to hear or absorb the truth. Thus, he responds to the difficulty the only way he knows how..... he tells her no and hopes she asks again someday, so he can tell her the truth.
From the text, I believe it's obvious he believes his daughter too young. A child can ask you something, and at times your response is something they cannot deal with. Most little girls want to marry their fathers.... they don't need to hear the grisly details of a place left behind. They don't need to share in the memories that are likely the worst her father will ever know. In addition, I don't think he would have wanted to see the response in his daughter's eyes.... daddies are most often seen as heroes. How hard would it be to see that gaze change..... to have your child see you in a different way?