If you are serious, here’s an explanation:
The Gemara in makos is discussing involuntary manslaughter cases, where a death is caused completely without malice, intent, or recklessness. The Mishna there is ONLY discussing the difference between criminal negligence, in which case the person is liable to exile, and accidental death, in which case person is exempt from criminal liability. In the scenario with the father, the act of hitting is not intended in any way to cause severe physical injury, nor is done with disregard for the child’s safety, and ONLY done for valid chinuch purposes (which would be evaluated in court). An example might be nudging a child into a pool of water to teach him to swim (a mitzva of chinuch), but then the child trips and hits his head on a stone and dies. With another person, the father would be forced into exile. With his son, if he can prove that he pushed him for the mitzva of chinuch, and that the force was not excessive or dangerous to the child, it will be a valid defense against liability. It’s a pity prize, though — the father lives on knowing he killed his son.
The defense is clearly rebuttable. If the father cannot prove he was engaged in a reasonable means of education, that could reasonably be expected to achieve the goals of education, then the next Mishna states that obviously the father is liable. If the father intended to cause serious injury, or acted with reckless forcefulness, he would be liable for murder. There is no absolute defense.
Thus, Rava’s classifying hitting a child for chinuch as a mitzva is ONLY brought as a rebuttable defense for a well-meaning but slightly negligent father. In this context, there’s no reason to interpret this as a mandatory approach to chinuch in general, but rather just adequate to be considered a “mitzva”, and thus a defense to manslaughter. But the father still acted negligently, otherwise he wouldn’t need the defense at all (it would be accidental death).
(This is the only way to understand Rava’s statement: Abba Shaul in the mishna had stated that performance of a mitzva is a defense to involuntary manslaughter. He says chopping wood is not ever a mitzva, and therefore if the ax head falls off and kills a bystander, the chopper is always liable to exile. Rava explains this, saying that even if the wood would be used for a mitzva, chopping itself is only a hechsher mitzva. This is because if you already have wood chopped, chopping more is not a mitzva; so if you don’t have, chopping wood is still not a mitzva. Ravina challenges Rava’s logic, saying that if applied to chinuch, if a son could learn without being hit, then hitting is never a mitzva. Rava finally brings the passuk to show that hitting could always be considered a mitzva IF the parent can show if was for the sake of education, even if there’s another way to educate. Again, in context, this is hardly an exhortation of parents to hit their children.)
Many other gemaras are consistent with this view that harsh chinuch measures are not permissible. The definition of “harsh” and “chinuch measures” will naturally adapt to what is normal in a particular society at a given time. That’s because the “foreseeability” of the harm will change depending on what a child’s normal reaction might be, which is heavily cultural.
In most countries today, children have certain rights under international, federal and state law that they have never had before. Any stranger can call child protective services on a rough parent or teacher, and every child knows this. This naturally influences what is perceived as “harsh” or acceptable.
Furthermore, developments in the fields of education and psychology have also changed our perception of chinuch and harshness. It’s not made-up sensitivities — it’s scientific knowledge that we did not have access to generations ago, much like the polio vaccine. Chochma bagoyim taamin. There’s no contradiction with Torah here.
Most radically changed is the structure of the family, going from closed and conformist, to individualistic and autonomous. A child has access to ideas, resources, and networks that were unfathomable to adults even a few years ago. If he doesn’t feel safe or understood at home, he has many, many options open to him. Therefore harsh chinuch no longer works in most cases. Even the Gemara and rishonim cautioned against pushing a child too far in the name of chinuch — today the boundary of “too far” is just much, much closer. One of many examples:
יורה דעה קמ:כ
אסור לאדם להכביד עולו על בניו
ולדקדק בכבודו עמהם שלא יביאם לידי מכשול אלא ימחול ויעלים עיניו מהם שהאב שמחל על כבודו כבודו מחול.
Consequently, in 2019, if you take a physical approach with the son in the hypothetical case, minyan attendance will be the least of your problems when he’s 14. What’s more likely is that he’ll continue to defy his parents, only he’ll hide it better and the material will become increasingly scandalous — and he’ll have access to internet. You’ll be “teaching” this already tight-lipped child to become savvy at hiding many worse things from his parents.