Hello mam, Firstly we will have to consult and examine what is the stain. Whether it is dental caries or calculus . If dental caries then filling has to be depending on extent of caries. If calculus then proper scaling and polishing has to be done.

Black stains can just be because of standing or could be cavity. Needs to be checked properly. If just stains do proper cleaning. If cavity then needs fillings.

Hello. We need to identify if these stains are decays are just tartar and calculus being stuck on teeth. Can u help us with a photo, As this would help us identify the problem.

Some medications also causes black discolouration on tooth. Baby sleeping with her sipper bottle keeping in her mouth for the while night can also cause caries in front tooth of children. If the water in your area is having more l flouride content can also cause chalk white to brown to black stains on the tooth. And poor oral hygiene is also the main cause of blackish discolouration. Rest, thum sucking habit will cause her teeth to protrude and not stain it. Make her stop it. Do visit your dentist near by. She is too young to undergo any vigorous treatments at this age. Please get her treated in time. Good luck.