optimally, child nodes on the "one" cardinality end of a relationship, should have a stripped down gui that makes them appear...

its been worked on to a limited degree

the best you can do so far, is to edit the subform config that shows the child that is in the "one" cardinality, and tell it to not allow "reference addition" or "reference deletion", but do allow "add new" and "edit child"

as a result of this, you will only ever see the outter subform fieldset with its title, and one contained fieldset at a time (either fieldset for editing the one child, or a fieldset for "filling out" one new child)

i know this isn't perfect, but in the short time i'm making the addition of some kind of access control system to be a higher priority

if you look at the code though, and search for different instances of the word fieldset, you can probably get this feature done yourself