Hmmm, you say you need two, and you say that "I was thinking of Chopin nocturne in C sharp minor and Chopin nocturne in é flat major. Is two Chopin pieces too much do you think?"
Yes, I think that two nocturnes is indeed much too much, and are much too similar. However, as one of the two pieces, the Chopin Nocturne in C-sharp minor (posthumous) would be wonderful.
For the other piece, "Sinfonia" from Bach's Partita No. 2 seems manageable, though that is probably on the difficult end. Perhaps better, as other posters have suggested, is Bach's Prelude No. 2 from Six Little Preludes. However, I would urge you to take a look at Prelude and Fugue in C minor from WTC Bk. 1, as this piece is fairly impressive (especially for a thirteen-year old) and may be manageable.
In sum, my recommendations are:
1. Chopin Nocturne in C-sharp minor (posthumous)
2. Sinfonia from Bach's Partita No. 2 / Prelude No. 2 from Six Little Preludes / Prelude in C minor from WTC Bk. 1.
I hope I have been of assistance.