I taught violin and music theory for a University prep program when I was in the later years of my undergrad.
A few things stuck out at me when I was doing this, and keep in mind--I had no classes on education theory at all at the time. I was just a good, gifted performer who had done some serious study of music theory. Music Theory emerges from the Philosophy of the time, so teaching about that is also teaching about the history of Western thought and also the social climate of the different periods of time that you are covering with music. Along with that comes the baggage of the Catholic church. The history of "Classical Music" is much aligned with the history of the Church.
So there's a lot to cover.
I learned a ton from this experience, and it was good for me, and -- I hope -- good for my students.
Some things I learned from this in no particular order:
In a one-to-one setting with a young student, as in really young, like 4-5 years old, where you are trying to teach difficult things, get down on your knees. That young kid is looking at an unfamiliar giant who is now telling them to do things they don't understand. They are still looking at you and wondering what the fuck is happening. Get to eye level with that kid. The kid will be far more focused than if they are looking at what they think of as a giant stranger.
Time yourself. If you are going to work on some really demanding physical and mental things with kids like learning to hear and play an instrument, give the kid a break every 10 minutes or so. The formula I ended up with at this gig was to do 10 minutes of supervised practice on instrument mechanics and then 5 minutes or so of theory.
Across a range of "talent" levels, I was able to produce pretty amazing progress with this method because you can keep a 5-year-old's interest for a lot longer if you are changing things up every so often.
I could do an hour or two with a young kid, where other teachers were losing steam and traction with the students after a half hour.
I never beat any kids, or hit them in any way. I grew up in that environment, and I thought it was awful as a kid. As an adult I find it even more repulsive. It's not, in my opinion, necessary for high achievement. And in my personal experience, it was detrimental.
In a classroom situation, I feel like you are sort of stuck. You have to present things as best you can and hope that everyone gets it. But you can have some creative control sometimes. I remember teaching one theory class to a wide range of kids. All of my students got locked into one history class at the prep school. So I had kids who were 5-6 years old and kids who were close to my own age at the time (I was 19, and the oldest students were 17).
That's tough, and I don't have anything to offer someone stuck in that situation. But it is possible to come up with a curriculum that can work for this assuming that everyone in the class in equally ignorant. It's just hard to do.
Don't be a male teaching any underage female past puberty one-on-one. Just don't do it. I never got into trouble, but I know many totally innocent people who have. I'm sure it works the other way. But there's a lot of emotion going on in pubescent kids, and they are likely to have confused romantic feelings about figures of authority who are not abusive. Just don't get close to that situation.
The last thing I want to say from my experience is that kids like to be treated with respect. That's part of the getting down on your knees thing. Another part of respect is to just assume that they will behave well and create an environment in which it makes sense for them to behave well. Talk to them like adults. Give them the freedom to fail and make mistakes.
They usually will not fail. They will usually step up to the challenge and do some amazing things.