This task is just an extension of spraying items with Windex and wiping them with paper towels. The glass helps them perfect their skills at wiping things clean. In preparation, you will want to have washed the car on the outside. The job of washing the vehicle is not assigned at this level, but it does not mean that they can’t help you do that before they clean the windows. 

Teach them to wipe up and down on the inside and side to side on the outside. This direction change will help them identify whether the streaks are on the inside or the outside of the window. 

Some sprays are ammonia free. Some manufacturers recommend using ammonia-free products on windows, especially when tinted. We really like the scent of some of the non-ammonia brands. Another good piece of advice is to ensure that the car is parked in the shade. The windows will streak badly in direct sunlight no matter how well you clean them.