Killing your own child is genetic suicide
And that's okay
Somebody else's suicide is none of my business. Somebody else's child is none of my business either. If a child wants to defend itself against its parent by hiring protection, that is its business, but not really my concern. This ability/willingness/decision to effectively protect itself against its parent might in some future society mark the distinction between childhood and adulthood.
We need to punish people who murder non-family-members, because if we were to fail to implement that law, then natural selection would eventually produce a breed of human that felt no compunction about killing other humans outside of their family.
In contrast, we as a society (and especially we as individuals considering what to do about strangers killing their own children) do not have a need to punish people who murder their own children, because if we were to fail to implement that law, then natural selection would nevertheless keep down the number of people who murdered their own children. While I might need to worry about my own parents, that's my business - thanks but no thanks, I don't need your help. If it gets bad, I'll take care of my problems with Mom and Dad.
The instinctive horror that you and I and everyone have about murdering our own children once born is the basis of the massive distinction that we make between the baby that we do not see (i.e. before birth) and the baby that we see (after birth). Our instincts are evidently keyed to the visual stimulus of the baby. This is why pro-life groups believe that showing photographs of aborted babies is effective. Indeed, it is somewhat effective in getting a response because the dismembered corpses of unborn infants look like infants who are victims of horrifying murder (and some number of pro-lifers may have been persuaded by such images).
But this horror itself is instinct, and that instinct arose no doubt through natural selection. The very existence of this instinct keeps down the practical problem of people killing their own children. (Which is, of course, really their own problem to begin with, genetically speaking.)
We have, curiously, discovered a loophole in our instincts: we are horrified by killing our born children (presumably because of the visual and tactile and olfactory stimuli that they provide, which normally creates a parent-child bond about as strong as what has held the Golden Gate Bridge together), and so rare is the parent (other than a desperate parent) who kills his or her own infant child. However, the same logic of natural selection, one might think, would dictate that we should be extremely reluctant to abort an unborn fetus. The loophole is that for the most part we cannot get any stimuli from the unborn fetus that would attach us to the fetus, and so many of us feel very little reluctance to kill these little infants of ours whom we have not yet seen or touched or smelled.
Significantly, the fetus starts visibly bulging the mother's belly at around three or four months, which, curious coincidence, is also about when we stop aborting our unborn children. The baby having made its existence unmistakably seen and felt, is now rather safer than before, though not entirely out of the woods.