Don't just turn your young student loose, but hold the rod with them and perform the cast together. With 15 feet of line out, lift the rod smartly and stop beside the child's ear. After the line unfurls to the rear, power the rod to the front and lower the rod. If they seem to be getting it, let them make a few casts by themselves. The key thing for adults to have when teaching youngsters is patience.
Good casting takes a long time.
Keep your practice sessions short, because kids are used to sound bites. I recommend 15 minutes. If your child is not with it that day, just put the rod away. I'm a firm believer that you should never apply too much force to big fish or little children. Mel Krieger helps a youngster improve her casting form.