Your little girl is probably grinding her teeth whch is a common habit in young children. Sometimes children who are under stress start grinding their teeth when they sleep, but often no cause is found for this habit. Teeth grinding is also known as 'bruxism' It is a very difficult habit to stop and can sometimes be so severe that children grind down some of their teeth especially their molar teeth and may need to be seen by a dentist.The good news is that this habit always stops when the permanent teeth start coming through at about the age of 6 or 7 years.