A child form is a Web Form that is in a one-to-many relationship. This DataPage receives a parameter from the query string and records it automatically. This way, all new submissions are associated with the correct parent record.
When creating a Submission Form for one-to-many relational DataPages, be sure to enable parameters and include the foreign key field into the form. Also, this field is typically hidden so your end-users cannot accidentally change the unique ID. Configure this field to receive this field as an external parameter. It may appear like this: