Paris Metro Pass

The easiest pass is the carnet, which is not a pass at all but simply a package of 10 tickets which you can hand around to your group. It does offer a substantial saving over individual ticket purchases. Each ticket is good on Metro and bus trips in the central district; travelling to other districts requires higher fares, such as from CdG airport into the centre. Passes tend to apply to specific time spans or require other documentation. Read all about it at http://www.ratp.fr/en/ratp/c_21879/tourists/ and www.parisbytrain.com
One small trip: You use a ticket to open the entry gate at the station. You are required to hold on to it until you have exited at your destination.

I don't know about child fares. But for the adults, a series of carnets will probably work best. That's because the weekly pass is good only from Monday through Sunday, not for any 7 day period (London's 7 day pass can be started any day, but Paris's can't). As stated above, a carnet (pronounced car-NAY) is just 10 tickets (costing about 2/3 the price of buying 10 individual tickets). They're shareable, so you just buy a new one and spread them among your group when you need more rides.

And if you plan on purchasing Metro tickets at any of the unmanned stations with ticket machines you'd better have a chip and pin credit card...nothing else will work.

Child fare tickets are for persons up to age 10 so your 11-year-old will have to pay adult fare. However, there is a one-day Ticket Jeunes pass for persons under 26 that is good on either Saturday or Sunday. For zones 1-3 it costs €3.65/day. It is undated and is only good on the day it is activated when a person takes his/her first ride. For the rest of your time in Paris just buy 10-ticket carnets as needed and share the tickets. Each ticket is good anywhere the metro goes (some lines end in zone 3) and on the RER in zone 1 (all of central Paris). It provides unlimited transfers until you exit the system. Each ticket allows bus transfers up to 90 minutes after boarding the first bus, but you cannot transfer between bus and Metro/RER on the same ticket. If you are going to travel beyond zone 1 (to Versailles, for example, which is in zone 4), buy single tickets each way.

Adding to my previous post. If you go to Versailles, you'll pay €3.35 each way. An alternative would be to buy one-day, zone 1-4 Mobilis passes for that day. They cost €10.85 and would cover all your rides all day long and you wouldn't have to use any tickets from a carnet either before or after you go to Versailles.

And don't let anyone "help" you buy tickets if you have to use a machine, especially at the Gare du Nord station. Better yet, when you are in a station with a real ticket window, stock up on carnets.

Thank you to all of you who responded. We will take your advice and go with carnets except for our under 26 yr. olds!