It was a summer baseball camp that I attended in high school where I was first formally taught how to slide. How many scabs and a jammed wrist could have been avoided if I was taught at a young age how to slide properly. Don't assume because kids can slide that they know how to slide. That's like assuming a player can hit because he can swing a bat. The most important slide to teach is the bent leg slide. The biggest problem with the bent leg slide is that kids tend to slide on their side. The proper and less painful way to slide is on your butt. The second problem is that kids often want to put their hands down as they slide. This can lead to jammed fingers and wrists.
A good way to teach a bent leg slide is to teach it the way you teach many skills, in steps.
Checkpoints for each step:
Unless you're stealing second, don't slide head first into second base. The chances of being stepped on and injured are too great. For younger players, the head first slide is not recommended for anything other than diving back to first. A head first slide puts you in a vulnerable position where it is easy to injure your hands, shoulders, and head. A good bent leg slide straight into the base is the best and safest way to get to the bag, protect yourself, and break up a double play.
Go to any youth baseball game and you'll see players who have scrapes all over their legs from sliding. Player's mistakenly think that a bent leg slide is performed by sliding on the side of the leg. The proper way is to slide on your butt. Practice sliding in grass and check the stains on your pants. If they are on the side of your leg, you're not sliding properly. Another common problem in sliding is putting your hand down during the slide. This is often the cause of jammed fingers, wrists, and sometimes shoulders. Get in the habit of sliding with your hands up. If you can remember to slide on your butt with your hands up, you will be sliding pain free from that point on.