For a child? I think black isn't a popular color for kids (9-12), so what would you envision?
What do you mean by "spy music?" Is it James Bond type stuff?
A mix kind of like James Bond/jazz/big bop/blues.
Do a tuxedo-style dress. Generally works well with that type of music, particularly with the little girl set and keeps it fun. You can do a girly-colour and still look Bond-ish,or you can a sparkly black for the "cutaway coat" part of it, skirt, and bow tie, and a white shirtfront if you want, and it still looks fun and young enough.
Thank you! She likes that tuxedo style. Better than my "cocktail" dress idea. :D