At that age, particularly if she's already in the school, I don't think you all have anything much to worry about. Never the less, you can coach her -- that if indeed an audition is required -- for her to treat it like she's taking a regular class (which it typically is) and not to get too nervous over it. Taking auditions is something that she can get familiar with and more comfortable with, even if the tummy butterflies never quite go away [which is a good thing].
Passing or not passing an audition is not necessarily a reflection on her ability, talent, or love of ballet and dance. Sometimes it's more of "Are we a good fit?"
She should try to have fun and be excited about it, rather then chewing her nails.
Those watching auditions often look for things beyond raw talent -- how well do they learn and take direction? Are they prepared? [On time, hair up, in clean leotard and tights, shoes.] Are their faces bright and excited?
Hope this helps and best wishes to your child for happy dancing!