add another delete record behavior. you can use the same variabel from the first delete record behavior that contains the ID for the parent record and select the relation column as the key column.
I'd like to second Neo's wish even though Jason has pointed out that this is already doable. Since this is a wish list, I'd like to wish that we not have to already be programmers and know these workarounds. It would be grand if there was a tool, utility, toggle, drop down, some other appropriate term, that would auto-magically make this happen. Probably not by version 3 but please consider it.