Assuming your primary key is an auto-enter field, you should be able to create child records without committing. What error are you getting?
One way is to make sure your primary key (parent) is generated upon commit and not creation.
The error I'm getting is "New Record/Request has been canceled. Do you wish to continue with the script?". This is not from the system like I first thought, but a result of an error in a script that I'm running. My apologies.
The parent record uses auto-assign for the key field and is empty on creation. I applied conditional formatting to the button which adds a new record to the child table, to hide that button until a value has been assigned to the parent record key. That actually did the trick and was hidden until I clicked on the portal which (a) committed the parent record and assigned that record a key and (b) revealed the button to allow the child table to be updated. So all is well [except for the scripting error that I have to run down.]
Thank you both very much for the reply. They helped jog a few brain cells.