Hello,
The tooth has deep decay.
Please visit a dentist the earliest you can, and get an xray of the tooth done.
Considering the age of the child.. if the next permanent tooth is about to erupt soon ( based on xray), you can go for extraction of the tooth.
Next Steps
Do not delay treatment too much. Though it's a milk tooth, it can cause food to get stuck there and affect adjacent teeth. and it can also sometimes affect the next permanent tooth that's going to erupt.
Health Tips
Rinse well after every meal
Keep the area clean
Practice brushing twice daily