She's nine? Be sure she's doing this for fun and not because she has her heart set on being spotted by a talent scout. Nine is a little young for mapping out career plans. Nine is also too young to start voice training, in many teachers' opinions. Nine IS fine for trying things out and enjoying herself.
If she's willing to do this just for fun, make sure she understands that the judges will tell her both good things and bad things about her performance. Unless she's willing to listen to the bad things, too, she's not ready for this sort of contest.
Then let her try, if she must. Her attitude at the end of the competition needs to be, "I'm glad I tried this!" Let her know you're glad, too.
(I DO hope the judges will take the contestants' ages into consideration in the way they respond!)