Just some random ideas:
Leave the door open a bit/a lot
Talking book playing softly or a recording of you or your DH reading a book to her
Spray some special protecting spray around the room before you leave her (DD had a little bottle of Monsters B Gone, which smelled a lot like tea tree oil and lavender in water )
A special item to make her brave like a bear or even a medal or dream catcher to hang over her bed.