The problem you have is that you cannot create a record and link it to a record that you haven't yet saved; because you do not yet know the Record ID. This can be achieved with some javascript or non-native solution.
You would need to contact me offline for help with the formula I suggested. Bear in mind that this formula doesn't require you to add the Parent; but it gives you the flexibility to add the Parent while remaining in the Child if you need to. It's 5.30 here, so tomorrow morning would be a good time to reach me. Click on my profile for all my contact info.
