If it is on tooth it could b decay go for filling with dentist. If it is on gums it's melanin pigmentation for which u need not to worry it's natural color

Hi..it could be a cavity. Show it to dentist once. Regards

If you could share her pic then it's easier to diagnose.  But rampant caries or nursing bottle syndrome is possible.  Stains due to iron syrups or multivitamin syrups possible too.  Plz show it to a competent doctor. Feel free to consult Laser Smile Care Centre anytime.  God bless u always.

There would  be multiple reason for this. she is on iron suppliments?  If yes it might be the reason or  if she is on bottle feeding, might be nursing bottle caries,  it can be pysiological also,must visit to nearby doctor to caught up with the correct diagnosis and treatment planning.

If you can post a picture it would be better to look at it and than say exactly

Hi ...The dark patch or stain if on teeth could be due to decay ..as suggested by fellow doctors,It may occur due to bottle feeding ,If it is on the gums. .1)gums naturally have pigmentation on them ..It is due to melanin ..for this nothing needs to be done.If at a later age she feels conscious she can do cosmetic gum treatments....2)If she is on Iron supplements also she will have patches of brown on gums and tongue too.If so talk to your physician and get alternate options.All the best.