Some parents never grow up. For one reason or another their emotional development stops somewhere. When a child is crying, other children usually ignore him, but his mother will come to his aid. But not all mothers. Young mothers or those whose emotional development was retarded for some reason will not. They never outgrow their selfish or insensitive childhood.
Children are incapable of rendering assistance and therefore have no use for empathy—it would only impede their own survival (evolutionarily speaking). They will react, in-kind sometimes, simply because it makes them uncomfortable, but not out of the more genuine concern that a mature parent might have.
We’re all selfish to some extent and therefore not fully mature; but those who abuse their children or neglect them are much further down the maturity/emotional development ladder than the rest of society. Jesus recognized this. It is why he said “those who will lose their life [unselfishness] will find it, and those who will save their life will lose it” (John 12:25 paraphrased).
A small selfish child is not terribly aware of his world—pretty ignorant. As we lose our selfishness we lose our ignorance and gain knowledge or understanding. Those we disagree with or fail to understand can be reconciled by losing our selfishness, or our focusing on our own hurt, in a word, being mature. We can then gain the knowledge or further understanding we would not otherwise get and find peace.