For this you will need to edit the registration page that is created by security assist.
you will need to delete the Dreamweaver Insert Record behavior that is applied, and replace it with the DataAssist insert record behaivior. This is covered in the Solution recipe on the security assist support page in the Modifying registration video:
(Note: This video uses encryption for the password field, you dont have to follow that part if you dont wish to encrypt the password).
Make sure that the insert record beahavior does not have a redirect set, and make note of the Session name that will be created in the store as section, this session will contain the id of the record that is inserted to be used in the foreign key column of the other tables.
Now you will need add the additional form elements to collect the data that will be store in the other tables.
After these elements are added, add another Data Assist Insert record behavior for the parent info table, bind the parent info columns to the parent info form elements, then add a third insert record behavior for the detail info table, binding the detail info columns to the detail info form elements. In both of these insert record behaviors, bind the foreign key columns that relate the info to the user to the session created by the user insert record behavior.
In the last insert record behavior, you can set the redirect to go to the login page.