It would be more appropriate to say, "Break a leg!" but that is something I simply can't say to my own child. She is a literal girl, after all. It will suffice, I think, to say,
"Although it's already been a long week of strep and post-ortho swollen cheeks (that remind us of your baby days) the show must go on. Good luck, sweet girl. We're very proud of you!!! And for goodness sake, whatever you do, don't break any legs."