What activities are there for two teenagers and two adults in Paris, France?

Q:
(14-16 years old) (40-50 years old)

A:
Tons of things to do.
Of course, there is the Eiffel tower, a great place to get a view of the whole city, watch the sunset, and enjoy being on top of the world.
Nortre Dame cathedral - go up on the bel towers and view the gargoyles, you can also (on smog free days) get a good view of the \Eiffel tower and Sacre Couer.
Sacre Coeuer up in Montmartre is an interesting church, you can go down to see the burial places of some, and up on a scramble around the top of the church. The steps are a good place to rest, eat a picnic lunch and watch people, then you can wander around the artist area and see artists at work in the square, have a coffee, and there are a ton of shops for tourists up there.
If the kids are into art, or have a period of history or french person they are interested in - the museums are fantastic. The Louvre and Orsay are must sees- you can easily set a time and place to meet and go off exploring on your own. The louvre has a great antiqities area for those interested in Egyptian sculpture, mummies, and the like. As well as lots of sculpture and paintings.
Go to the Rodin museum and take your picture by "the thinker.
An interesting if macabre place is the catacombs.
At night you can wander around the latin quarter and buy crepes form street vendors (my daughter always liked the nutella ones)
You can do a boat ride on the Seine at night, see the city lit up and get a riverside point of view of the main sites.
The arc de Triomphe has a 5:00 ceremony everyday for the unknown soldier, with veterans from many wars attending, and from up on top you get good views of the Eiffel tower, and dwn the Champs Elysees. You can walk down the Champ Elysees towards the Seine, and when you get tired hop a metro to the place de concord and ride the ferris wheel.
There are usually caricature artists near the main tourist sites if you want to get a family one done to remember your visit.
A side trip to Versailles can be interesting - in addition to the palace and all the luxury inside, there is an enormous park with gardens and fountains to explore. Sundays there is a fountain show which is spectacular.

You can also do day trips via train to Chartres (cathedral painted by Monet and where St Joan met her end) , Reims (cathedral where kings crowned and champagne winery tours - thru their caves, very interesting), to Giverney where Monet's house is, or up to Deauville on the Normandy coast or a day tour to the D-day beaches and towns. Pont du Hoc is amazing for showing the destructive inpact of bombs on the landscape.