To children, particularly little children, their parents are everything. They recognize on an instinctive level that they are screwed without at least one parent around to take care of them. This extends not only on the most basic level of food and shelter, but on an emotional level. Little children have a construct of what they instinctively know their parent(s) should be - unconditional love, protection from all the monsters of world both real and imagined, and the person(s) they can trust beyond anyone else in the world. When this construct isn't realized, children try whatever they feel they need to in order to fulfill it, feeling it is a failure on their part that their parents don't meet this construct, because in their minds parents are supposed to act a certain way and when they don't, children immediately assume all the blame for the equation failing. This happens on two levels and in two different ways.
The first is by simple omission on a parent's part - i.e. some form of neglect. This can take the shape of outright physical and emotional neglect and disconnect, or the subtler version of using unrealistic expectations as a measuring stick against which approval and attention is doled out.
The second is by actual actions - i.e. verbal or physical abuse. While neglect is bad enough, this is even worse as every part of the parent construct is shattered, not just the protection aspect. Because how could a parent who is supposed to be the most sympathetic and understanding and loving person in a child's life, be able to put that aside and cause the physical and emotional pain of corporal punishment? Children don't have the reasoning capacity to interpret this as anything but themselves being the most utter failures imaginable.
I found this picture on Who Am I Without You's blog, and it is perfect in every sense. Yes children feel the physical pain of a corporal punishment, but it is this image of the emotional pain they grow through that is the real consequence. They don't understand. They just don't. They're left feeling alone, confused, and no amount of explanation or make-up "hug" (that happens in 'punishment' vs 'random' abuse) erases the starkness of them not only being in pain and crying and desperate, but that it was their parent who put them there. Their parent who supposed to protect them from that sort of thing. This is further not helped by the fact that in case of what is conveniently defined as "punishment" instead of "abuse" by those who hold with using corporal punishment, that the child is told it is their fault! In their mind and in their extremely limited comprehension, they are utterly worthless. After all, they're told to be nice to animals/pets and clearly they must be worse. And the kicker is, they can't even articulate that, not even to themselves. They just feel it on an intrinsic and instinctive level. They can't explain they didn't deliberately disobey or even have any comprehension that they'd done anything wrong to begin with. And they can't explain the devastation afterwards. They're just left crying for far more reasons than they can possibly comprehend but that hurt worse than the actual sting of being smacked or hit. They failed their part of the equation so horribly that that most basic parent construct clearly has gotten revoked.
But they still look for it. Still try to achieve their part in a desperate attempt to get that most basic emotional need that their parent construct contains. Try to get that look of approval, that hug that feels genuine, the sense that they can trust. And so eventually they "behave". Other people tell their parents how they can't believe "how well your children are behaved". But all that's happened is the children are broken. Yes they're trained; but they're trained in the sense that a beaten puppy is trained. And ultimately, no amount of approval or praise will mitigate that they know that all it takes is one mis-step, one mistake, one thing that isn't approved of to bring the facade of the parent construct crashing down again.
A child's psychology is not something to taken lightly or dismissed, and
the resulting and lasting emotional damage is often profound. Most parents who choose to use corporal punishment wouldn't dream of holding down a puppy and "spanking" it for often upwards of two minutes. But they do that to their little boy or girl who has barely no more comprehension than that puppy. They're a child. A little human being only looking to be loved and understood, and when they feel like they're blindsided with the harshness of a physical punishment and can't explain why they did whatever it was and why they have no clue why it was wrong or the expectation that that should know just makes no sense, it creates a lasting impression and trauma that bites far deeper than any immediate result. Particularly as they get older and the instinctual recognition of what a parent should be and how they're not becomes more cognitive. That relational construct gets destroyed, true trust in the parent from the child gets destroyed forever. I think this is particularly true in the case of religious based corporal punishment as it is always presented to the child very specifically that it is their fault and an extreme over-expectation in behavior is demanded, as opposed to alcohol/drug/mental illness induced physical punishment which can be explained away as a child gets older and gains understanding of the external circumstances that it really isn't their fault in the slightest. In the former (religious/conviction) based, it never stops not being the child's fault entirely and the parents never see that what they did was wrong. The emotional sting and devastation never goes away.
Even as children grow up and to an extent or completely become able to intellectual grasp all of what I laid out, they're left, often even as young adults, permanently feeling like that little 3y/o, crying on the inside and sometimes on the outside over the loss and absence of that most intrinsic parent construct, and wondering what exactly it was they could have done to deserve being denied it. The moment one recognizes the loss of that relationship and that it will never be achieved, is profound, and can be as painful as dealing with the physical loss of one's parents. Because in a sense one has come to realize that they have lost their parents - the parents that their little self looked and kept hoping for and that they will never find. And the grief of recognizing that loss never completely goes away.
Tell me that hitting toddlers hardly old enough to walk when they don't sit still during a nearly 2hr church service doesn't instill some level of fear and disconnect that is unnatural between parent and child at that age.
And tell me there isn't a deep devastation that this child is feeling when this is the result, not of a disappointment or overtired frustration or any of the other normal things children get sad about, but because she's just been dragged preemptively out of a store or church and swiftly and harshly "spanked" on their bare bottom for something she could mostly explain but is neither given the remotest chance nor have the emotional vocabulary to know to do so because she 'knows' at this point it would only be perceived as "talking back" and get her in bigger trouble. Tell me this doesn't create a significant and permanent breakdown of at the very least a healthy relationship between child and parent, and nearly always the destruction of any true emotional relationship whatsoever. Tell me any actual trust exists when a child is reduced to this on a regular basis by their parent, even in the name of love. They end up not trusting that love and ultimately don't even know what real love means if they've learned to define it as "I love you enough to hit you."