Filter child field options by a parent field
I have done this type of thing before. I did it by creating a map (array) of what 'scenes' belong to which 'shows' and sent the javascript array to the page with a hook_form_alter. I then used javascript/jquery to listen for the change event on the 'show' dropdown which then hid or displayed the the corresponding 'scenes' using the array as reference. It could also be done via ajax if you preferred.