I have a flight from St. Petersburg, Russia to Atlanta, GA with a 7 hour layover in Paris. I am travelling with two children. All of us are US citizens. We would like to get out of the airport and do some sightseeing. Is it worth it? Do we need to clear customs?
Assuming you are landing at CDG, you can try to catch the fast RER B train into Paris which should take you to St Michel Notre Dame in about 30 minutes. From there the Notre Dame cathedral is a stone's throw (and a lovely bridge crossing) away. IMHO that's well worth visiting and doesn't require too much time.
The calculations the provided by the others in answers and comments seem about right, giving you a couple hours to queue up, see the cathedral, eat French crêpes in the nearby streets, and get back on the RER. Should you have time to spare you can head north for Hôtel de Ville, see the building and then go back to the RER at St Michel Notre Dame.
I think two hours is plenty to arrive before take off. Even for long haul. Still, it takes time to get out of the airport so about 1.5hrs from landing to the city and you need to be at the bus stop three hours before your flight so tops you'd have 7-4.5=2.5 possibly only two hours. With two children, this begins to sound quite risky.
I landed in Paris 5:40 AM and had a connection at 1:10 PM, so 7.5 hours layover. My luggage was checked through to the connecting flight, so within 20 minutes of leaving the plane I was outside. I took the Le Bus, Route 2, direct to Etoile (Arche de Triumph), 17 Euro and took about 45 minutes to get there (OK, it was Sunday, so quicker than usual). From that point, you have a few hours to walk around. Make your way down the Champs de Eleysees to Place Concorde, and there you can see the outside of the Louvre and the famous gardens. Then walk to the Opera and there is the Rossey Bus, which runs every 15 minutes direct to the terminal, for 12 Euro. SO you can easily have 3 hours in Paris, not including the bus rides, which are also sightseeing. For the price of 2 bus tickets, it's well worth it!