I was nine and eleven during my first two sojourns in Paris, so I can relate a little to what your kids may experience. This was a very long time ago but some things haven't changed all that much.
My biggest disappointment was Versailles, which my mom had talked up quite a lot. When I revisited the palace thirty-five years later, I bumped into an American family with two kids of about the same age I'd been on my first visit -- and they were sulking around, thoroughly not enjoying it.
I was fascinated by the metro, the tiny elevators, the Eiffel Tower, the campground concierge's
Solex moped, the electrical and plumbing hardware -- mechanical things that were dramatically different from anything I might see at home. My mom's enthusiasm for churches, especially rose windows, rubbed off a bit, and I enjoyed the gargoyles of Notre Dame, the stonework, and the dramatically huge ancient buildings overall.
The toy sailboats on the Grand Bassin of the Luxembourg Gardens were always fun, as were the carousels. I'm sure we visited the
Musée des Arts et Métiers, which was one of the numerous "push-button museums" we kids looked forward to in our travels. In fact I remember the Foucault's Pendulum on display there.
I didn't get what Napoleon's Tomb was all about but I was impressed by its grandeur.
We didn't visit the Catacombes. I still haven't, but I'm pretty sure I'll be rectifying that oversight this summer when I'll be in Paris with my twelve- and fifteen-year-old grandkids.
Discovering eclairs was a delight, but now that first-rate pastries are often available in the States, your kids might not notice. Paris was where my mother introduced me to Grenadine (pomegranate syrup) in water, and I drank it everywhere we could find it in Europe. I was thrilled to eat octopus for the first time, too.
Language was never a problem for my family, as my mother was pretty fluent in French and my father could get by. I learned the language too, so I don't have problems communicating. But English is so widespread now that you'll have little trouble, except perhaps when you're trying to remember which way to go on the metro. Most of the lines end at polysyllabic stations whose names I find hard to remember even as a Francophone -- was it Aubervilliers (now known as Front Populaire) or Porte de Versailles? Jet-lag makes these names evaporate like morning dew.
You'll have a great time. Enjoy!