If you want the child row to refer to the parent row, yes, you’d need to set a column in the child row to a reference to the parent; a reference to the parent is not added automatically.
If you are creating the child row from the parent row, the parent’s key column value is what you need to set the child row’s parent Ref column. For instance, if the child table has a column named Parent of type Ref, in the action for the parent row that adds the new child row, set Parent to [_THISROW].