When practicing sliding try using a large piece of cardboard placed on grass. Have the kids start thier slide hitting the piece of cardboard. This causes the cardboard to slide on the grass instead of the kid so there is less chance for injury do to abrasion or maybe snagging a cleat or shoe in the grass. Use a stick of some kind as sort of limbo bar and have them slide under it to make sure they stay as low as possible. Make sure they practice good form(slide more on your back than side and keep the hands up and back).