To be fair to Karl and Cosmic, neither of them are saying it is good to kill their children, or that they would do so, or that they should not be punished if they did. I have no doubt that they love their children, and I doubt they would kill or abuse them. From what I can tell they are just saying they will leave punishment up to Allah and that human society shouldn't interfere. Cosmic has now taken this position for not just child abuse, but for all human-made law. I ask them to correct anything I just said.
This position seems to run against any sense of protecting the innocent from wrongdoing. If Allah doesn't exist (my view), then this position leads to absolutely no personal accountability for ones actions. If Allah does exist (the majority view here) then this position leads to an after the fact punishment for wrongdoing rather than an impetus to prevent it.
Are there not numerous calls in the Quran for us to actively protect the innocent from harm, or have I been reading that in? If you saw a man torturing a child, or killing children, would you not feel compelled to stop them? Does the Quran not instruct you to stop them? Pretty much every other moral code I have come across does.