I assume there was (and partially still is) a very strict taboo on intrafamilial murder and abuse. It's very hard to hide a murder nowadays, but just looking at abuse, we know there are a lot of unreported cases even in a modern society. It must have been much worse in more 'primitive' societies.

I would assume that it happened but was passed off as an accident and not murder. That is a man runs crying to town "my wife is dead!", cries at the funeral, and gets community support as a man grieving his wife - she is dead because he poisoned her, or ensured she would get hit by those falling rocks that look like an accident, but if you don't have modern investigation you have no way of knowing that. (it can go the other way too of course)

There are lots of ways to kill. Some will be investigated more than others.

Who would report it? If parent kill their child they are unlikely to, if someone else kills their child they are likely to.

> From the evidence I have gathered from primitive societies

What evidence are you referring to? Also, again "unthinkable" is different from it not happening. In fact, it makes it even more likely to be covered up, if people don't care to investigate, or otherwise want to know.

On the other hand, I think it was common in a lot of places for a long time for families/clans to settle disputes internally - if someone was killed and everyone agreed it was justice, then it wouldn't end up reported/remarkable. This is probably the category that most intrafamily disputes would fall into.

A lot of old fashioned nursery rhymes and children's stories are about evil step-parents?