We will be in Paris for a week in July with grown kids. What is the best deal in terms of metro? Should we buy a card or the 10 ticket carnet? Would we need photos for the cards? Thanks much
The Passe Navigo at 20 euros lasts specifically from Monday through Sunday and does need a photo and 5 euro set-up fee. If that roughly fits the period of your visit, it's a good deal and can be supplemented with a carnet for a day or two "on the outside." Youths under age 26 can buy a Jeune Weekend ticket good all day Saturday or Sunday for about 3.65 each.
I think a couple carnets would be more than enough. They can be divided among the family and you never have to buy more than you need. I think you will find Paris very walkable and when it's so hot descending into the stuffy metro isn't much more appealing than walking to your destination.
If your week falls Mon-Sun, I would get the Navigo Decouverte just for the incredible convenience. If you plan on using it in from CDG or back to CDG or have a couple of day trips out of town then charging it for 5 zones might make sense. You can also charge individual journeys on to it right before trips outside of zone. e.g. if going to Versailles you could charge it within 3 hours for the trip out and do the same on return at a machine (have coins) This parcours charge saves a bit.
If your week is not mostly within one calendar week then using carnet of ordinary tickets is probably the best bet. Paris is best viewed on foot so you can use the metro to get to a distant spot like Montmartre or the Eiffel Tower or whatever and then walk walk in the area; we also would often metro out to such a spot and then walk back. Paris is not large and it repays the stroll through its lovely streets.
If your adult kids are 25 or under they can get a day pass for Sat or Sun called a ticket jeune which is a big money saver -- about half the cost of a Mobilis day pass. I think the ticket jeune is about 3.25 for Paris.
also keep in mind, the Metro is underground so you won't see any of the sights. For a lot of trips it might be just as easy to walk. Don't take the map as gospel, stations are not as far apart or as orderly as they seem. Use subway for medium-length trips. A good map of the city will also show Metro stops aboveground so you can see if it's worth it.