I have always used a "slip and slide" or similar device to teach sliding. I line the slide up from the base back toward the runner, sprinkle cheap bath powder (no talc) on the end of slide and have the kids take off their shoes.

Make sure the kids don't run at full speed and have two coaches at the end of the slide in case someone slips.

I do it at the end of practice so the kids go home smelling great. Thanks John, I'm sure the mom's appreciate that!