If child record is not found by link criteria the form is at a new record and the Parent ID is passed to the relevant foreign key field on the child form and a new record can be entered.
You don't say how you are doing this so I will explain a "best" practice.
When I use the technique of opening a popup form for data entry in addition to using the WHERE option of the OpenForm method, I pass the FK value in the OpenArgs argument. Then, in the popup form's BeforeInsert event, I copy the OpenArgs value to the FK field on the form. This means that my code does not dirty the new record. When your code dirties the new or current record, you run the risk of creating "empty" records unless your validation code in the form's BeforeUpdate event prevents that. You can also end up giving the user strange (to his mind) error messages since he KNOWS that he didn't type anything so why are you complaining that he is trying to close the form without adding/saving a record?
I also use the model property of the new form to stop the code in the calling form at the point where the popup opens. That forces the user to close the popup before going back to the calling form. It also gives me a place to requery the main form to pick up any change made by the popup.
There are a couple of other acceptable methods such as setting the default property of the FK control on the popup to the value passed by the OpenArgs.
Both of these methods handle the situation where the popup might need to add more than one record. All records added will have the FK properly populated but not until it is necessary.
I also agree with
@gemma-the-husky 's point about the 1-1 relationship. It is quite unlikely that you actually have one and it is probably better to put the data into the main table.