I'd like to echo everything that BG said, I thought her post was really spot on (the area around H&M, Monoprix - if you're daughter likes make-up she NEEDS to look at the Bourjois selection at Monoprix, she'll pay tripple for it at Sephora in the US). I also bought the book she mentioned, it's a good resource.
Also, when you're rushing to and from sights, you'll pass amazing little shops every day. Let her go in every once and awhile. I really applaud your decision to let your 15 year old shop in Paris, I think it will be a very memorable experience for her. Sights every day can be grueling. Some of my favorite memories of Paris (though I'm 25) was the day my boyfriend and I spent the afternoon shopping -it was more like just a relaxed day like we'd spend at home, with lots of laughing, neighborhood exploration, and fun.
I respecfully disagree with some of the previous posts. 3 hours is like, well THREE HOURS! Your daughter will see plenty of great sights, day after day of sights, but those 3 hours will please and pacify her - and keep her from feeling like she missed out on something that she is interested in. Plus-when I wear the things I bought in Paris here in the US, people and friends are always noticing and asking; which leads into some great conversations about Paris in which I am able to talk about the amazing sights, ect. The necklace or sweater she gets will just be an outlet for her to start talking about the Mona Lisa, or the Orsay =) Have a nice trip!!