Children are not individuals that can take anything and forget about it. If a parent chooses to smack a kid, and I'm not even talking about actually hurting it, he/she should realise what an impact it has on a kid. Children don't forgive and forget.
That is the whole point Rags, when you smack a child whether it hurts or not the main aim is to instill in the childs memory that what they were doing to receive the punishment in the first place is wrong so every time they see the object or activity the remember the smack they relieved and refrain from doing it. The same way an adult mind tells us not to touch a fire because it is hot even if we have never been burnt before earlier in our life either our Barents have taught us that it is hot and we have remembered or been punished for touching something hot etc etc.

The point is they link that activity with either the shock of the smack or the pain if the smack is given that way and so they don't do it because they know that it is associated with pain or shock.

That's where there should be boundaries. Smacking kids around shouldn't be a regular punishment (like it was in my home when I was a kid). If a kid is outright arrogant and totally out of control, ignoring and neglecting every house rule and behaves totally inapropriate and doesn't realise the consequences of its behaviour, a smack cán have a good effect. It makes the kid realise "woh, I really did something that is NOT tolerated around here". But if you're going to smack your kids for every little thing they do wrong, that's just giving the wrong example. If I was fighting with my brothers when I was young, my parents would break up the "fight", tell us we were doing something wrong, and then smack us around. That is just plain hypocrisy and it has made me so that I don't respect my parents nearly as much as I (probably) should.
Yes this is true there is boundries it is just up to the individual parents themselves whether to respect those boundries or not. When a child is doing something that may injure or hurt or be life threatining then yes a smack should be in order it enhances the effect of the reprimand of the action but it should also be followed by a stern talk to the child no matter what the age you need to stop them calm them down and explain why they got a smack or was punished for what they just did.
Also, as mentioned earlier, it's not the "hurting" part that can have serious effects on their minds, but the simple act of violence as a response to something that is out of order. Like I said, smacking a young kid when it has purposely done something really bad that hurts you as a parent can be a sad but effective means TO MAKE A POINT ON RARE OCCASIONS, MAYBE. But you should never make a habit out of it.
That's true you should never make a habit of smacking and use it as a last measure, i myself have different steps to deal with my children depending on the scale of what they are doing wrong it usually starts with me asking nicely to stop what they are doing and then i say it again but in a serious tone in my voice the third step is to warn the child after counting to three the will receive timeout or a smack but usually i count further to three because as soon as they hear me start to count they get their act together and do what i asked them or stop. Yes it does affect the child in the long run i was smacked as a child hell my mother went through some wooden spoons with me haha and i smack my children when it is required but if your were not to smack a child then i can only assume that when that child becomes a adult and has children that he/she will also not smack their own children depending on the personality.

My parents never bruised me or anything, and if they have it would have been an accident, but the smacking around (and my dad's general violent attitude) and that threatening raised hand has had an effect on me, without any doubt. My dad's violent attitude is a feature of him I can't forgive him for and one of the reasons why I hate him.
I remember my dad used to get his leather belt fold it grab the looped end and the open end and pull both ends to make a cracking sound holy hell it always scared the crap out of us hahaha and i just remembered he always had a grimace that he would pull that scared us well but he never hit us with the belt even after he cracked it.

Anyway in answer to the original question i don't think it should be criminally convicted because than at least 50% of the world or whatever country would be in jail or have criminal convictions. I think it is utterly ridiculous that you can't smack your child in public or get scolded for reprimanding you child in public and funnily enough the ones that do scold or stare are usually the ones without kids and when we catch people watching we approach them and ask the nicely "Do you have Kids" and the answer is usually no and we say 'Then you can't judge' which is true unless you have kids you can't judge or scold parents for how they raise their own children, either that or we ask them if they want to take them home and the quite quickly say no hahaha it is funny to watch of course we would never send our kids home with someone else but it is amusing.

What i found the biggest help is to go to the child's perspective and act from there especially with tantrums so i ask myself why are they throwing a tantrum and then act from there.