Dennis @ PerfectForms
As shown in this help guide page: /Documentation/manual/html/?parent_instance_ids.htm both the instance ID and the parent instance ID can be accessed in the Form Designer. Select an object that will display the instance ID and on the Properties bar, make a selection from the Special value list.
You could use a Show Form behavior object to open the parent and pass a custom parameter: /Documentation/manual/html/?fl_show_form.htm.
You should also be able to use a Set Field in Other Instance. As indicated in the help guide section referenced here, it is NOT necessary to pass the parent instance ID to the child, it is meta-data (I was wrong earlier).