An easy workaround of sorts might be to let her wear it but tell her she will need a "specially magic card" for her to be able to wear it. She'll have to use this magic card to "make this grownup's MB work with her wearing it."
Then, ask for a KTTW (Key To the World) card for her when you check in at your resort or go to guest relations if staying off-property. The card is free for the asking as you may know.