How are child records related to parent records when using subforms?

I am new to Zoho Creator and I am having a problem understanding the link between child records and parent records when using a subform. I want to be able to initially import the child records (rather than have to enter them all in the interface) and still have them display in the subforms when editing a parent record. I can only get the ones I enter directly into Zoho to appear in the subform.

I import the child records with the correct ID for the appropriate parent record and they load into the correct form without errors. Yet when I look for them under the subform only records entered directly appear.

Am I trying to do something that cannot be done?

Thanks

Kevin