More good news: 12-yr-olds and teenagers in general are the perfect candidates for altered state techniques like guided imagery and hypnosis – they take to it like ducks to water – probably because most teens are already soaking in a trance-inducing hormone soup in their bloodstreams (that's my personal hunch – no hard data to back this up that I know of).
And a twelve-year-old is old enough to use an adult audio program. So I'm recommending she listen to Successful Surgery, once or twice a day (or as much as she wants) before her surgery. This should help a lot with her anxiety. She can also bring it into the O.R. with her and have it played during surgery; and after surgery, the affirmations track is designed to help her through recovery.
We wish her the best. Let us all know how she does.