K.S.
HI-
I have had a bit of the same experience recently with my 9 mo old son. His stains are dark, almost black & were in the grooves of the teeth, so no matter how well & often I brushed, it wasn't budging. However, I bought a battery powered brush & after a few brushings, it came off! I also am a Registered Dental Hygienist who practiced in Pediatrics for 2 yrs & I spoke with my dentist about the stains. It seems that if it is the dark stains, then it is insoluble ferric salts (iron deposits) that stain the plaque. Even though you brush religiously, there is still plaque in the mouth, so the iron deposits stain the plaque & are a booger to get off. Try the spin brush & see how it works for ya. Otherwise, if it is really bothersome, then she is not too young to get a cleaning from a Pediatric dentist office.
Good luck!