If the dentist has given her the all clear then this is not something to get too concerned with at the moment.
If she was affecting tooth enamel or surfaces then I would be worried. If she continues this when she is older then she can have a mouth guard to wear when sleeping.
Ensure that her bedtime routine has her thoroughly relaxed when sleeping but it is not a deficiency. Having said that - ensure she is drinking plenty of milk and having fruit and vegetables daily to help build healthy teeth and bones as she grows. Brush the teeth well at night and avoid candy and soda drinks during the day.
Have a look at this article for interest: http://att.iparenting.com/preschoolers/teeth.htm
And do make sure that she continues with her regular dental visits.
Regards
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