Unless you're stealing second, don't slide head first into second base. The chances of being stepped on and injured are too great.
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.