This question is for clarity only: You say you get the "required record" message when the parent table's PK field is not an autonumber. The question is whether your "BeforeInsert" event is the parent form's event or the sub-form's event.
I cannot tell you with absolute certainty, but I have seen evidence that the presence of an autonumber causes the PK field to be filled at a different time than a non-autonumber PK field. I suspect that you are running afoul of that difference.
Also, though it is trickier to type and also is a matter of style, you could make the frm.[ID] reference be Me.Parent.[ID] - but note this only works on sub-forms and would fail (regardless of how it is coded) if you ever tried this on a form that IS a parent and is not a sub-form.
You could take the guesswork out of the problem in several ways, including not allowing the sub-form to be visible unless/until you key in your (non-autonumber) value to the ID field and, in that field's .LostFocus routine, go back to enable the option to show the sub-form. I'm vaguely remembering that for "normal" Access, you avoid relational integrity issues with child forms because Access does an implicit SAVE on a parent record linked to a child form that has just been given focus. However, your code would prevent that child form from gaining focus. At some point the timing becomes too complex.
Final note: If you REALLY want the child form to not allow entry until the parent is defined, just declare relational integrity between the related fields in the tables or queries that drive the parent and child forms. Then trap the error in the form if you violate RI. To be honest, while I'm sure it is well defined, I don't recall the exact order of events for this case, and I think that event order is what is tripping up your process.
