@Robert... If understand you correctly...   You want  a master table.. a child composite table.. and ensure the child has one and only one row.???? 

Create a view and a button on the master (aka parent) form. 

Hide the default table/relationship link by setting it Display if to false. ... Then add a View for the child.   This shows the child but does not let you add.

Put logic in the button to display only if the child count is zero.   For the formula.. create a Child and assign the current master as the owner.