As for the whole "kids can't be reasoned with" line, that is a crock.
There are two situations here. The first is kids over the age of 3 or 4. Kids at this age and up absolutely can be reasoned with. They are actually famous for going back and forth like freaking Socrates with their parents. And you know what? Sometimes they're even right and the parents are wrong. If you build trust with the kids from this stage, they'll be a lot more likely to listen to you later.
The other situation is the kids under that age, toddlers and infants. These kids indeed cannot be reasoned with. And that is all the more reason to not fucking hit them. If they are getting into seriously dangerous situations at this age, that is on the parent. No amount of hitting will get a 2 year old to not want to put his finger in the electrical socket. It's up to the parents to make sure he can't get to it in the first place. Yes, you can talk to them about it and explain things, and start the process of giving them good reasons for doing or not doing whatever, but at this age their brains are only just starting to be able to handle the very concept of cause and effect. When you hit a 2 year old, they don't tie that in to whatever action the just took. All they process is "daddy just betrayed me and hurt me."