Hi Andrew,
I would expect that user should be responsible for deleting records that they don't want. Using a childsaved attribute would be a neat solution if your users were not keeping track of their records.
I've implemented this with a dialog before that asks for the mandatory information before creating the new case and then using a custom dialog (workflow) activity to copy the N:N relationship to the new case. This way it forces the user to provide information and if they cancel the dialog then no case is created. You can then provide a hyperlink to the new case at the end of the dialog.
hth,
Scott