Simply tell your mum that you don't think that you've really outgrown the DSi, and that you'd really like to keep it because you enjoy it.
As far as your voice and other puberty changes go - well, I'm 13, and my voice is breaking too; it's perfectly normal (well, I suppose anything to do with me can't be normal, actually
), if you want to chat about anything else, please feel free to
PM me (click my name on the left of this post, and "Send a Private Message for Rachael98")