Thanks all!
She was funny. They're both on stage a lot, and have been since they were little. They're probably in 3-5 stage productions a year. This is the first time, though, that she wasn't singing with either a group of kids with a backing track or over a CD. This was for a "Broadway after School" program that she does, which is mostly kids singing over CDs.
A week ago she thought we were good until I forced her to practice with me, and then she realized that singing with a live musician is a bit different than anything she'd done. We then practiced for about an hour a day until it got where it needed to be.
Neither of my daughters are ever nervous before a show, but this time as I was setting up she came to me, "what do I do if I forget the words? What if I sing the chorus too many times? What if..."
I told her just to relax, and reminded her that we worked out the cues, and I would play based on her, so when she dropped the mic down, it was solo time, and if she forgot a verse to just repeat the last verse, etc. She nailed it. As I had expected, the practicing paid off.
She made history in the program because it was the first time they'd ever had real, live, music.