This is much better than having adults lecture to them. I believe that the K-3 children should be taught not to take pills from strangers, and to only take medicine from their parents when they are sick. Older kids can emphasize these points, and they can model responsible behavior in the face of peer pressure to experiment with drugs. Your daughter and her classmates could put together little skits that could demonstrate how to handle a pushy drug dealer or party where some kids are trying drugs. They might be able to do a puppet show or make up a song they could teach the younger children.
Fourth graders are very clever at creating ways of helping younger children learn something.