According to me kids should be introduced to team sports like football first. This way they will build stamina as well as learn to play in team and learn from others. Running is an individual sport which can make a child feel lonely and bored, and he may lose interest soon.
I think after they are comfortable with participating in an individual sport, introduce them to shorter form of running like 400m and 800m. Once they are in high school, encourage them to participate in 5k and 10k races.