My hitting materials are geared toward hitters who are, and are capable of, taking hitting more seriously. In general, that means hitters who are in at least 3rd grade and more often 5th grade.
However, I am often asked what to teach a hitter, or a coach, who is just getting started and isn't ready to go too deep into mechanics.
A Natural Progression
Hitters are able to understand, and do, different things at different ages. As a result, I tailor what I teach, and how I teach it, to the age of the student.
The first thing to work on with young kids is the concept of Rotation; you need to make sure that the hitter is powering their swing with the rotation of their hips and shoulders and not just their arms. The telltale of an Arm-y Swing is one where the bat and arms move but the shoulders don't move.
The George Brett Drill and the Neck Drill are two simple, basic drills that are designed to help young hitters get a feel for powering their swing with their hips and shoulders and not just their arms and developing a good first move. When doing either drill, you want to make sure that the hitter's hands are back and their front elbow is up, away from their chest and in the plane of their shoulders in a position of good Alignment.
At this age, I like to keep the lower body simple and keep any stride to a minimum. I like my hitters to take a small step forward toward the pitcher. I also do not think that it's necessary to teach kids to squish the bug in order to get their hips rotating. In fact, in my experience teaching squishing the bug tends to hurt hip rotation more than it helps it.
Assuming a kid is powering their swing with the Rotation of their hips and shoulders, and not their arms, the main thing to work on when a kid is in Coach Pitch is their Swing Plane and not screwing it up completely by creating an Excessive Uppercut.
A Slight Uppercut (5 to 10 degree) is what you are going for and, if you are doing coach pitch and not tee ball or machine pitch, then it's important that you not pitch in a way that will cause your hitters to develop a gigantic uppercut (30 to 45 degree).
Elementary School Players (1st Through 4th Grade)
When working with elementary school players, you should still keep things fairly simple when it comes to hitting mechanics. Assuming they are powering their swing with Rotation, I would mostly leave them alone unless they are exhibiting one of several common problems.
There are a number of critical problems that you see in younger hitters that can keep them from being able to hit the ball at all, and that should be fixed ASAP.
Posture problems are a major problem in young hitters and something to pay attention to from the start. Too many kids set up in upright, lock-legged, un-athletic stances with their feet too close together, which keeps them from being able to swing and adjust with anything but their arms. The result is arm-y swings and dropping the hands. It's important that kids be taught about the important relationship between Posture and hitting at a young age.
Lunging can often be caused by being overly aggressive (which is often the result of poor coaching or the use of problematic hitting cues). One way to fix a problem with lunging is with the cue, "Don't go get the ball, wait it come to you," and to stay away from cues like, "Attack the ball." Lunging can also be caused by putting the tee too far forward (too close to the pitcher). The proper position for the tee is at the heel of the front foot.
Chopping Down on the Ball, and general swing plane issues, is something that some kids do naturally. Unfortunately, some coaches actually encourage things like chopping down on the ball out of the belief that it's harder to make a play on a ground ball or that chopping down on the ball with create backspin and will cause the ball to carry farther. The truth is that it's much more effective to swing with a slight uppercut which matches the plane of the swing to the plane of the pitch and maximizes the odds that you will hit the ball square.
Gigantic Uppercuts can result from coaches pitching the ball at too steep of a trajectory. Instead of pitching underhand or from one knee, they stand up straight and lob the ball, which causes the ball to fall pretty much vertically downward onto the plate. In my experience, kids can quickly learn to handle faster, flatter pitches and that is what coaches should be throwing them both during batting practice and during coach pitch games. The Target Tee Drill is a good drill to use with kids this age to give them a sense of the trajectory that they are going for.
As I explain in greater detail in my Bat Sizing Guide, non-wood bats are organized according to their length, barrel diameter, and drop, with the drop being the difference between the length and the weight of the bat. For example, a 29 inch, 18 ounce bat would be a -11. When buying a bat for a grade school hitter -- and especially when buying a bat for a kid who is at most average when it comes to size and strength -- one should err on the side of lighter, higher-drop bats (e.g. -10 or -11). Even with a big, strong kid, I would be reluctant to have them swing anything heavier than a -9 bat at this age. Yes, you will be giving up some pop, but you also may be reducing the likelihood that your hitter will develop a problem with Bat Drag. I say this because, although I am not sure, I believe I may have contributed to my sons' problems with Bat Drag by having them use heavier bats (-7s) before they were strong enough to handle them.