The blackening of the teeth is actually Dental caries
, which is caused because of many reasons, excessive intake of sugar, improper oral hygiene
, usually in the kids its because they sleep while/ after consuming milk with sugar at night. Most of the times we don't take take of kids teeth but it is most important to take care for many reasons most important ones are that the permanent tooth at that particular area also gets affected directly or indirectly if the milk tooth is damaged.
Nothing to worry about, consult your dentist
, restorations can be easily done, if there is no pain and if pain is present then also treatment known as pulpectomy can be done if required.
Always remember "Precaution is better than cure...".
Make it a point that not only your kid but everyone at home brush their teeth twice daily, n kids should use a special toothpaste which is specifically for kids, for small kids parents should help/assist child while brushing.