Sure, your children can be altruists, but would raising your children have highest marginal return? You only "win" by the amount of altruism your child has above the substitute child. So if you're really good at indoctrinating children with altruism, you would better exploit your comparative advantage by spending your time indoctrinating other people's children while their parents do the non-altruistic tasks of changing diapers, etc. Children are an efficient mechanism for spreading your genes, but not the most efficient mechanism for spreading your memes.