There are a lot of family activities during the day they can do together... Disney trivia, pool games, table tennis, shuffleboard, board games, etc... they do other fun family activities with the cruise staff, like putt putt in the atrium, or leap frog games, and learning to draw mickey....and then there is always the family pool, and sunning on deck 10. They can also go to the movies together, internet cafe together.
7 day has a lot of of behind the scenes stuff, that technically is for adults... but I don't think anyone would mide, or notice if they went to a seminar or two together.
at night they have family karaoke, family dance time, family comedy shows, family game-show types of activites after dinner.
then there is always the offship beach stuff and shopping to do together, and excursions they could take together.
the 17 year old will not be able to go to the adult clubs, I have never seen them card or ID people, but technically its 18 yrs and up. Same thing for the adult pool area.
As far as missing the teen club I can't really say... I would say the 17 year old should at least give it a try, to make a few friends... in case the 20 year old wants to do some of the adult-club stuff at night.
They will have lots to keep themselves occupied. You'll be surprised how time flys, even on sea days, after your meals, and sunning, and doing the cruise activities throughout the day.