There are taxis at the airport for 6 people; you can also order them from G7 -- if returning to CDG order the return taxi two days in advance making clear you need a van for 6. You pay 5.50 extra per person over 4 -- so you will pay an extra 11 euros for your two extras. You don't order them for the airport -- just go to the taxi line ignoring anyone who approaches you offering a taxi. You order them for the return.
The NL is only available for locals with French banks.
If you use the train instead of cabs into the city and do most of your travel e.g. Versailles etc and lots of hopping around Paris the first week then the Navigo Decouverte makes sense. Each person needs a head shot 25/30 mm and there is no child fare. (it is not a tourist product but tourists can buy it). It covers everything in the Ile de France -- buses, metros, RER, trams, funnicular -- so super convenient and you will break even if you use it to come in from the airport. If you do this you probably need to get a Navigo Easy for the days of the second week; another weekly pass would not come close to breaking even unless you used the train to the airport, then it might be worth doing.
If you use a cab from the airport then get everyone a Navigo Easy card for 2 Euros each when you get to Paris -- at your nearest metro and put 9 metro tickets on each for 2.50 each If your child is 9 or younger tickets are half price for them; everyone else pays full fare. If you will use buses, they use a different ticket for 2 Euro. We don't because we use the metro
This may cover the travel you do but if you run out, you just add more tickets.