It's nursing bottle caries.visit a pedodontist and get teeth filled or pulpectomy done based on pulpal involvement. You can consult through this for prescription of medication and further details

Hi It looks like your niece is having Early childhood caries, which is cavity affecting primary teeth in children less than 5 years. Ideally you should be visiting a pediatric dentist ( a dentist who specialises in children) and do the required treatment. By just looking at the photo it looks like she will be requiring pulp therapy for her teeth followed by crowns.

I understood your concern and noted the problem as well. I need to ask few more questions to make a detailed prescription and treatment plan. please consult me on practo or in person so i am able to help you better.

Usually in few kids all teeth gets damaged due to keeping food in their mouth for long time or due to having milk or any sugar substances in their mouth which is not cleaned immediately.. Actually ur niece is having caries which has progressed till nerve .. basically tooth is lik pressure cooker.. what ever decay is deep and infected it goes into the teeth tip area .. as the infection wants to drain it dsnt hv place so as it finds place to drain in the form of pus.. still not to worry it’s better you get her treated immediately though they r milk teeth they might effect the growth of permanent teeth.

Hello Procedure is needed to treat it properly

Looks like she needs Root canal. But my advise visit the dentist or send your X ray for opinion