Medical term for grinding teeth is bruxism and it's very common in children most of children outgrow it.
Reasons are malalign teeth, stress, teething
Next Steps
please visit a dentist to see whether there is anychipping or unusual wear and tear of enamel which can cause sensitivity.and any muscle soreness. depending upon condition doctor may suggest use of night guard.
Health Tips
if the child is under stress due to school, or some change in routine try to speak to the child and ease her stress.