I just need to hear you sing, that's all! You can sing pretty much anything for your audition. As long as you can do so decently and make it sound like it matches the character's age, or ALMOST match the character's age, then it's fair game.

(Twinkle is 11.)

(I also encourage you to read up on Twinkle's personality, which is in the description of her regular speaking role above, before considering auditioning as her singing voice, even if you aren't going to audition for her regular speaking voice.)