Growing up, I was a very naughty child. I got smacked a lot, and my step dad would often tell me how lucky I was for it. Why? Because whereas I got smacked, he got a leather belt across his backside. In our culture of cuddle merchants, the concept of punishing children, especially physically, causes many to recoil. It is even considered by many to be "abuse".
A "slap on the wrist" is the description usually given to an extremely lenient punishment. You burgle a house and get off with nothing but a small fine, that's considered a "slap on the wrist". A slap on the wrist is used to describe a small punishment for a large crime because a slap on the wrist is a small punishment for a large crime. And yet even a slap on the wrist is more than some people ever receive. At least, from their parents.
The world is a much less forgiving teacher. Whatever you don't teach your children while they are young, someone, or something, will teach them when they are old. Sometimes, the punishment is "small". You don't teach your children a work ethic, oh well, they'll be poor for a while until they finally get off their butt and do something. Other times, the punishment is much larger. They fall in with the wrong crowd and deprive themselves of wonderful opportunities. They bully others until those people stand up to them. They abuse their bodies with sexual immorality, drugs, even just not eating healthily, and their body pays them back.
And perhaps they do something more sinister. They commit a bigger crime and end up in jail. They get caught up with a worse crowd and get seriously hurt, maybe even permanently injured. Or they do something more dangerous and die. All of this because their mothers, fathers, aunts, uncles, and all other responsible adults were not willing, or were not able, to drive out this bad behaviour. That is why, as the Bible says, to spare the rod, or the belt, or even the hand, is hating the child. If you don't teach your children the "easy" way when they are young, they will learn the hard way, when it's far too late for a second chance.