There are two main theories as to what causes teeth grinding (also referred to as “bruxism”).
One theory as to the cause involves a psychological component. Stress (a new environment, divorce, changes at school, etc.) can influence children to grind their teeth. Another theory relates to pressure in the inner ear at night.
If there are pressure changes, similar to what you might experience in an airplane, the child will grind by moving his jaw to relieve this pressure.