This calculated field returns a comma-separated list with the components of the current issue's sub-tasks.
Create a new Calculated Text Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional.
Select Advanced text as parsing mode.
Add the field to the relevant view screens.
Remember: All calculated fields are read-only and cannot be manually updated on a create or edit screen.
Check the results
Once configured and added to an issue screen, the result will look like this: