There is not "parent" and "child" form relationship, there is "Owner" and "Owned" forms. You really need to use this relationship if you need application integrity.
As to passing and data and any cooperation between forms, this relatioship just does not matter. It could be any forms of the same assembly.
I insist on using "my" most robust method based on some interface implemented on one of the form. Please see below. There are few different approached. You can find it all in this discussion:
How to copy all the items between listboxes in two forms[
^].
—SA