Unfortunately, i don't have an account to reply (I have attempted however, I am receiving this message ' User registration is currently not allowed.').
Ive been reading on the website that there is a pass called Navigo Decouverte. I am hoping that the entire family will be eligible to purchase this pass and that it will be valid to Gretz-Armainvilliers. Can I also please ask if we are able to buy the navigo decouverte as soon as we land at CDG and if it will be valid to our holiday rental? Your help so far has put my mind at ease as I had asked on tripadvisors forum and i was advised by pm that it was not a good idea to stay in Gretz-Armainvilliers without a car.
Navigo Decouverte is valid to Gretz-Armainvilliers and you can buy the pass at CDG airport train stations upon arrival. Keep in mind that the Navigo pass is valid Monday to Sunday, so if you arrive Friday, you can't buy a week pass for immediate use (current week passes are on sale only up until Thursday midnight), but can put day passes onto the Navigo card to tide you over to Monday.
If you are an Uber ride-hailing user, you should be able to order rides quite easily in the Paris area that can take you where public transport doesn't cover, but trains+buses have excellent coverage in France and Navigo makes it easy in terms of paying fares.