If you have enabled the Email Respondent option from the Form widget configuration, the portal users will get the email along with details (as per the Email template).
By clicking on the Submit button to add a new record (add form), the portal users get an email with the form data.
The Email templates are according to the form entity. In the case of the parent-child relationship form, the template and link of the record are according to the parent and child entity.
The portal users will get the Email as per the new entity records submission (Add Form).