Sensitivity is caused by open end of nerve of teeth, you should brush with sensitivity toothpaste like sensodyne, you can apply alum or fatkiri over the sensitive area, visit your dentist for this purpose.
Hello Mr. lybrate-user
first visit to dentist for the treatment. In grossly decayed tooth pulpectomy is done. Second thing don't worry about the permanent teeth. Milk teeth doesn't harm the permanent teeth.
There are some measures you may try...
Brush twice with homoeopathic toothpaste heklalava
Massage your gums with turmeric and salt twice daily
Take vitamin C supplements
And consult for proper homoeopathic treatment..