Make a "garage" out of a cardboard box and tell the boys when they are done the cars need to be parked in the garage. How old are they? Could they understand a "demotion program" if they get done playing and dont park the cars in the garage, the cars that are left out get taken away. If they really like their cars and want to play with them for the rest of the day, they'll learn quick to put them in the garage or.....bubye.