Work in a controlled environment like a backyard or school blacktop. Preschoolers are most likely too young to be on the road with the possibility of oncoming cars. Also try to teach on a level surface, as hills may be too difficult for the average pre school child.
Have all the children wear helmets. Even if they are not going to be riding that day, they need to get used to the feeling of helmets. Emphasize the importance of helmets not just because wearing a helmet is the law, but because it can protect your head if you ever fall off.
Let the kids ride on a tricycle to learn the experience of bicycling. Let a few do it at a time, and have a very controlled course. This isn't just about riding, but riding safely. Have the child ride to a designated spot not too far away, then turn around and come back, all while maintaining control and moderate speed.
Have some children ride on a bicycle with training wheels if any of them are old enough. Teach about the concept of breaking smoothly when they need to slow down or stop.
Set up a street crossing scenario by putting markers on the ground to imitate the edges of the road. Have the preschooler ride up to the edge, get off the bike (or tricycle), look both ways (left, right, then left again) then push the bike across the street.