It is easier to buy in Paris at any metro station because there will be a long line for the RER ticket booth and a taxi is pretty cost effective for 4 people BUT luggage often takes a long time and you do save 62 or so Euro by using your ND into the city. So this is not an unreasonable strategy (and you DO NOT NEED TO BE PRESENT to buy the ND.
I have purchased Navigo Decouverte cards for other people many times and assembled so that they are ready when friends or family arrives. There is no requirement to get them in person. You can buy the kit and put it together later or even have a couple of blanks on hand for some future unknown guest -- you then put their picture on it and name, assemble it with the gummed flab and then put the weekly pass on on a machine in the metro station. No ID is required, there is no vetting of the applicant or whatever -- you just get the card kit, assemble it (a clerk might put it together for you if you look clueless, they are nice and there is no line). and put the pass on it. You need a head shot 25/30 mm; you can print on your computer and bring with you. Passport photos are a bit too large. You do NOT sign the card, you print your name in the space on the card for it and the card has a stick spot to attach the photo before closing the gummed flap to seal it. You can load the card with the pass before you assemble it with the picture BUT you cannot use it because if stopped and it has not been assembled you are fined for not having a valid ticket.
to put the pass on the card, you place it on the pink swirly spot on the machine and follow the instructions on the machine. The machine takes US credit cards (just ignore the request for a pin) and it will tell you when the pass has been put on and you can take the card back. Keep the receipt. On one occasion our charge didn't take and we had the information clerk load the pass with our receipt in hand. If you buy it at an RER window the clerk will put the passes on even if she doesn't assemble the cards for you. Each transaction is done separately i.e. 4 charges on the credit card.
You cannot put a monthly pass on with a US chip/sig credit card on the machine because the amount exceeds the Euro limit for a card without pin in the European system. In that case Apple pay on your phone works or you can go to the information clerk with your card and they will load it. If you buy at the RER window then this is not an issue; the clerk can handle the card and get your signature.