Boy or girl? I think the assumption is going to be "boy" unless you specify otherwise, but I'd recommend different things for different sexes...

For boys, I'd probably start with games. Maybe Python and PyGame? I've heard pretty good things about it. JavaScript makes a good first language as well, since all you need is a web browser to make it run.

For girls, I'd do something webby with a social component. If you could write a server, maybe set up some sort of framework where she and all her friends could send things to each other, sorta like real-time Pownce? Actually, now that I mention it, I kinda wanna try implementing this, since it could be kinda cool.

For both sexes, maybe take a look at Alice from Carnegie Mellon? http://www.alice.org/