Hitting is frequently referred to by sports writers and authorities as one of the hardest things to do in all sports. You can improve your odds of success by learning and executing the appropriate (a) mechanics, and (b) mental approach. The vast majority of humans are not born with the proper mechanics, so developing it requires a lot of practice in order to build the needed muscle memory. For kids, keep it simple by starting with a few basics, then build-on it over time.
REMEMBER: a player won't hit well if they are stiff and tense, so be supportive of all the players!!! It's not easy to hit the ball, and it's definitely not easy to hit it in front of a bunch of fans with the game on the line.
Here are some basic steps to help build proper mechanics (with pictures below of the pros hitting home runs to match):
Control Point 1: RELAX & FOCUS eyes focused on the pitcher's release point
Control Point 2: LOAD hand's back, hips 'closed' (you DON'T need a bid leg lift)!
Control Point 3: STRIDE/LAND front foot down, hands back, hips 'closed'
Control Point 4: CONTACT hips square to pitcher, hands IN FRONT of back elbow, hands slightly higher than barrel, on ball-of-back-foot with heel in the air
Control Point 5: EXTENSION arms are fully extended (typically towards the pitcher or center-field) with barrel of bat at chin level
Control Point 6: FOLLOW THROUGH nice and balanced without falling over
Note: find some video of your favorite players hitting home runs, run it in slow-motion, and notice the similarities!
Debuggin' a Swing: Some typical issues to watch for: (A) before contact, the batter's back elbow travels towards the pitcher more than the bat (esp. on contact), and/or (b) the batter is trying to land their front foot and make contact at the same time, and/or (C) the batter is skipping control points 3-4-5 and going straight to an upper-cut 'follow through' when they should be getting to proper 'contact' stance and getting their arms 'extended' after contact (e.g., with their arms extended toward the pitcher / center-field with bat tip at chin hieght).
REALLY WANT TO GET BETTER ??? Take a couple hitting lessons (parents and kids); it's invaluable!