How can a tap have a shock value if it doesn't cause any pain? How does a non-painful tap cause a child to be so shocked that they immediately stop what they're doing?
Again, isolating/rejecting/excluding a child is not the only option which I have said before. It's not smacking/timeouts or nothing.
"And if you are lucky enough to have a family circumstance where you can constantly supervise and laid back children Good for you."
Again, the suggestion that people who don't smack are only able to avoid smacking because they are "lucky" and have laid back children. This is just not true. My DS is active, curious, persistent, strong willed and can be challenging - he isn't a mild meek little thing who just sits and plays. However, he knows where the boundaries are because they have been consistently enforced since he could move. He used to like to hit the TV when he was little, but doesn't do so at all now, because everytime he did it I would remove him, tell him that isn't what we do and distract him. It took countless repetitions but now he wouldn't dream of it.
As for the issue about not being able to constantly supervise young children - as a parent, supervising them is the job above everything else. Different children obviously need different amounts of supervision and different home situations will make that easier or harder. If a child has shown they are likely to repeatedly hit their younger/disabled sibling, then I wouldn't leave them alone together where that could happen. One of them would need to come with me wherever I was so that I could keep an eye on them.
I disagree with you that hitting them teaches them that it causes pain to others. Especially with younger children, precisely because they have not yet developed a full sense of empathy. All hitting them teaches them is that you hitting them hurts! And potentially to be afraid of you to some degree depending on how often, how hard and how intimidating you are when you hit. It doesn't teach them to care about the fact that hitting others hurts, even if they are able to understand that it would hurt others on an intellectual level. Plus, how does this opinion square with your assertion that a tap doesn't hurt anyway?
You said one very true statement, RJae:
"They push boundaries all the time. They are programmed to do that."
On this I completely agree. The role of a parent is to consistently and kindly (without physical chastisement) enforce boundaries whilst children are learning what is acceptable and what is not.