I had good English teachers for the most part. So there isn't anything really that I could say I wish I was taught. And I'm not a parent, so I can't say anything about any experiences from that perspective. Also, I was--and still am--really good at spelling. And I don't want that to sound like "Hey, look at me, I'm smart!" It's just been one of my strong suits (now something like chemistry I'm absolutely terrible at).
However, there are some grammatical/spelling mistakes that I see often that really grate on me. The biggest one is probably whenever people write/type could of/would of/should of. IT'S COULD HAVE/WOULD HAVE/SHOULD HAVE, JUST LOOK AT THE CONTRACTIONS BECAUSE THEY END IN "'VE." Sorry, that one really gets on my nerves.
I don't know what really young kids like 5-12 years old use these days, but I would think a good old-fashioned whiteboard is servicable. I remember back when I was really young those projectors that used a lamp to display those laminated sheets were in a lot of classrooms still (not the projectors that are hooked up and can display stuff from computers).
Kids learn in different ways, so you may have to work slightly differently with each kid. But an important thing is to not teach kids simple memorization. When I was learning some Spanish in high school, this was somewhat of an issue. Yeah, I could remember certain words for the rest of the year, but later on I'd just forget a bunch of stuff. You have to teach kids the basic grammar rules too, and why things are the way they are. When I did Spanish in college my teachers were more effective in teaching me because I was taught more about why words were the way they were as opposed to how they were, especially in Spanish II when my teacher would sometimes talk about the roots of the Spanish words and how some of them related to their English counterparts. When it came to vocabulary, there was also much more effective groupings to study, such as things that dealt specifically with hospitals/healthcare, banking, sports, food, etc. (Which reminds me, I've really been slacking on my Spanish practice ever since I graduated from my university...) Anyways, doing a similar thing with English ought to help.