I hope that how you feel is the worst of it. Hopefully you can get something that helps the cough and anything else you are dealing with or might deal with.
Im so sorry it prevented the kid from doing the show. You had hoped to be able to get thru today in the clear. I know you both are upset by this and the timing. What the dance people offered to do for her and you is so touching. That will help her and you heal.
It tough to understand why stuff happens but all you can do is take this setback and find a path forward that will let her be the best she can be and show off what skills she has.
Feel better!!