Hello..
It seems she is not able to brush her teeth properly. You have to keep a check on how she is brushing her teeth. Children below 7 years old should preferably be guided by parents and not left alone to brush their teeth.
Next Steps
Take her to a Dentist and get her teeth cleaning (scaling) done.
Health Tips
Don't try to remove any deposits, if visible on teeth through forceful brushing. It can damage her teeth as well as gums.