If after the permanent restoration/ filling there is severe pain then it means that the infection has gone deep inside the pulp chamber, which is not coming out and getting collected.
The amount of infection present is difficult to know unless one checks the patient clinically or you share the radiograph of that tooth.
Next Steps
The only treatment to save the tooth in this case is Root Canal Treatment. Patient can get a second opinion also, as checking and diagnosing the problem clinically is very important.
Health Tips
The milk teeth of children are very precious and you should always try to save them, as they maintain space for the permanent teeth to come in the Dental arches. Otherwise the alignment of permanent teeth can spoil and later on patient may require braces.