When you let suburban American children loose in a city--especially in a European city--you are bound to discover a vast realm of knowledge they do not have.
Sure, you expect a language barrier in Paris. (And, to a lesser extent, in London, where even common sentences like "mind the gap between the train and the platform" will elicit endless repetitions of the phrase "mind the gap" in an approximated accent.)
But there is also an urban-language barrier, both more unexpected and more awkward to manage. Here is a partial list of things suburban children do not know--and that you will not be able to teach them in a few short weeks, no matter how much you try.
Apartment house etiquette. Such as: Do not shout down the central stairwell after your running sister. Do not run down the central stairwell. Do not sing annoying songs loudly while brushing your teeth and looking out the open window into the common courtyard. Do not stomp like a heard of elephants down the hall; there are people living below you. Do not use your outdoor voice, even in a fight; there are people living all around you. Do not practice your tap dancing, run races, escalate your voice to ever higher pitches, or--heaven help us--walk as if your feet are made of lead: There. Are. People. Living. Below. You.
Sidewalk etiquette. Such as: You and your sister cannot hold hands with me and your father and walk four-abreast on a sidewalk that is one meter wide. When there are oncoming people, you have to choose one side of the sidewalk or the other And. Stay. On. It. so that they can pass you. That wandering thing you do, as if you were meandering through a field of flowers, doesn't really work on urban pavement.
Museum etiquette. Such as: Do not make audible yawning noises when you walk into a room full of Picassos. Also, do not touch the thin little chains that are meant to separate the people from the art.
Dirt. As in: it is everywhere. Please keep your hands to yourselves. No. They will not. They will run their hands along building walls as they walk, along fences, along subway tunnel walls, along benches, around signs, along tops of poles, along any vertical surface meant to keep people in or out, along any horizontal surface that is approximately their height. Even if you explain what that disgusting smell is in the subway tunnels, they will still forget and run their hands along the walls. The grime of the city will cover them from head to foot. (They will, however, quickly acclimate to being forced to bathe every single night without fail.)
On the other hand, there are lots of urban things they will easily learn, and some of them are invaluable life skills.
Negotiating public transportation. Including, how to read a subway map and figure out the most efficient route from Here to Where I Want to Be. How to manage the subway ticket gates--in multiple languages and different systems. And how to keep track of subway tickets (these--the children who cannot keep track of their own coats in winter!--can hang onto innumerable tiny slips of paper that let them on and off miraculous underground trains).
Buying fresh food daily. Frankly, this one is easy. You just have to choose what kind of croissant you want for breakfast and then walk to the corner boulangerie/patisserie to purchase it. Or pick your fruit and vegetables and stop at the market that's halfway between home and the Underground. This is connected to learning to eat a baguette, chevre, and sundried tomato spread at 10pm when you have finally returned from the day's adventures home for dinner, which is also a very pleasant thing to learn.
Toilets. As in: go when you can. You may never get used to paying for the privilege, but your kids will quickly get used to taking advantage of the opportunity when a public toilet pops up. (Hint, if you're ever in Paris with kids: the nicest free public toilets around are in the round park just before the Place de la Concorde walking down the Champs Elysee from the Arc de Triumph; the most disgusting ones you will pay for the privilege of using are in Tuileries Gardens.)
And, through it all, they will get to see some amazing things--and you will get to see those things through their eyes, which is even more fun. Moments like this (in the modern art gallery of the Pompidou Center) are priceless:
Even if they cannot remember after ten days of reminders that they need to quit stomping through the apartment. There are people living below us.