Auto-populate Lookup field to link back to 'parent' form?

Auto-populate Lookup field to link back to 'parent' form?

I have a long, complex form that I want to split into multiple Child forms, linked back to the Parent form using a hidden Lookup field that automatically gets populated via a URL from the Parent form.
I would need to populate a Notes field in the Parent form that can pop open the Child form including the Parent form ID.
Is this possible? Any tips?