As one of the Custom Parameters you are passing to the child, pass the instance ID of the Parent. In the child use a Set Field in Other Instance behavior object. Specify the form, Custom parameters, etc, and use the parent’s Instance ID to specify the parent.
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).