I would recommend a modified approach. Since the Users are usually adding Child records first, have a button which Adds the Child Record, Saves it and automatically Redirects the User into the Record in EDIT mode. This way you have your Record ID#. Then when the User needs to add the Parent-Record from inside the Child; it will launch in a new window; the User can populate the Parent Record and Save it; and it will auto-join to the Child. Then your User can Save the Child record.
If you are tracking [Date Created] and [Last Modified] as your time-frame for the calls; this should still work just fine since your [Last Modified] will pickup the last save action on the Child and the [Date Created] would be the instance they added the Child.
