Ditch the bags first at the hotel, for sure. Even if you can get into some of the rooms, try to resist the temptation to take a nap if you can. Most people adjust faster to the time change if they keep going that first day, maybe have an early dinner, then crash and try to sleep through the first night. The teens and grandpa will likely handle it differently.
Adrenaline can keep you going if you keep moving and stay outside. You could take a Seine cruise, starting near the Eiffel Tower. The link here is to a company that offers one-hour cruises every half hour. There are others also, probably also leaving from near the ET. This would keep them outside and awake, and give them a memorable orientation to the city.
Or you can try to get up the tower if you can stand waiting in line. The kids might make it on foot to the first or second level, which provide great views with much less wait time than using the elevators. Again, memorable orientation.
I'd avoid museums, even the Invalides which might appeal to teens, that first day. Too tiring. I've never taken the sewer tour. The question I'd ask is whether the smell and "gross" factor will keep them awake even though they're in a dark confined space for awhile. Only trying it will tell! ;)