You will each need to have your own Navigo Easy Pass (cost 2 euros). (Cardboard tickets are no longer sold.) As of Jan. 1, the possibility of buying a carnet of ten tickets at a discount no longer exists, so buying more tickets than you need doesn't seem like a good idea. I have my Navigo Easy Pass loaded onto my iPhone's Wallet, but I had the actual card first, so I am not sure it is possible to bypass that original 2 euro charge. The Bonjour RATP app for your phones (if you will have your phones) also may be a way to buy the Metro tickets.
In any case, the "day" passes/week passes, etc. would not be advantageous for you, and there is no longer a "pass" of individual tickets, so you will just be buying as many individual tickets as you need. It is a different (more expensive) ticket from the airport, but guessing there will be someone there to explain this upon your arrival.
