In the spirit of Landsburg's book, I have been meaning to remark that at the playground, there is a spontaneous polycentrism. Parents have different rules for what is safe/permissible for their kids, and generally speaking parents allow a situation to be resolved in the home court of the offending party.
For example, today Clark decided to go up to a kid on the swings and grab him. The outraged kid (who was much bigger) was getting ready to push him away. By this point I had left the bench (where I was reading the WSJ) and yelling, "Clark! Hey, leave him alone!"
My yelling alerted the mother of the kid on the swing, and she yelled, "Jimmy, just wait a minute."
Not the Cuban Missile Crisis, I grant you. But yet another point for private law.