Grinding of teeth is commonly referred to as Bruxism which does not have any apparent reason. Very often it is associated with worms but it is not true. Nothing needs to be done for this and it subsides on its own over a period of time. However, if it is very severe you may contact your dentist to prevent any possible damage to the teeth. Also if the child is going through subconscious stress, you may see a Child Psychologist.