So, was she supposed to create the song, or would playing it have been enough (assuming it was correctly credited?). I too think it's pretty darn impressive that she can play a song she's probably heard a few times on the piano, so I would emphasize that - what a skill!! As far as plagiarizing, I would go in with the assumption that she didn't know this was wrong. My ds10 still is a bit confused on the subject - how to include facts, but put them in your own words... what exactly needs to be credited? I mean, unless you're doing new research, isn't most of what we learn coming from SOME other source? Especially for gifted kids, and maybe kids like my ds who are very literal, this can be HARD! And even dd18, who is a writer and very creative, has worried about inadvertently "lifting" some idea since she reads nearly constantly. So, I guess my point is that I would be very, very positive and in fact, I'd probably take the blame myself ("we should have talked to you about this; I'm really sorry, honey"). Keep emphasizing what an amazing player she is and what a great skill that is. Talk to her about creating her own music, if she'd like. This isn't the advice I'd give for an older child, but at five or six, I think that the most important thing is to not discourage her and to keep the creative spark alive.
Theresa