What a fun trip! I've taken my daughter to Paris a few times, and here are a few of her favorites sights:
Notre Dame: we took a tour with Paris Walks and we all learned a lot. There's a playground in the garden behind the church which is a great place to blow off steam. Then, you can go to Berthillon for ice cream, and entertainment on the bridge going to the island.
Chocolate Tour: La Cuisine de Paris has some fun options.
Concert at Sainte Chapelle: ClassicTic had tickets at reasonable prices.
The Louvre: we used the ATM kiosks to get right in, even on a busy day. We also had an agenda that she set for us; an hour - 1.5 hours was manageable for us. I studied the map ahead of time.
L'Aquarium de Paris: wonderfully air conditioned for hot days! My kiddo would have gone here every day if we let her.
Deyrolle: A taxidermy museum ... sounds strange, but it really is one of the most unusual museums I've ever been to. Great for kids who love animals. It's located in a large park and there's also a small zoo close-by.
Just walking around Paris is fun, as you know. There are parks and carousels everywhere. We'd usually stop for a glass of wine for us and a small bowl of ice cream for her around 4:00, which was a nice way to break up the day. We also did "treasure hunts" to find croissant shops, the oldest tree in Paris (you can learn more in Rick's Paris book), and gargoyles.
We're not big shoppers but if you are, Galeries Lafayette hosts fashion shows that I've been told are lots of fun.
I also taught my daughter a few French phrases, such as saying, "Hello, how are you?" when she walked into a shop. This was a great way to meet shop keepers and learn more about the shop than just walking in as we do at home.
Have fun!