Traveling to Paris for 5 days

A day trip to Normandy's D-Day beaches is a full day, you will want to go through a tour company instead of trying to do it on your own for just a day. You won't see everything, but it can be done.

For Champagne, you take the high speed (TGV) train to Reims, which gets you there in 45 minutes from the Paris Gare de l'Est station. You have to reserve the train in advance, and you most likely will also want to reserve the champagne cave tour in advance to get one in English.
I'm guessing the teens will want to do the D-Day beaches tour instead of the champagne caves tour.

I'd spend the whole time in Paris. You can't see and do everything Paris has to offer even in 5 days! With a group of 8 including teens, I'd recommend going to www.homeaway.com/search/keywords:paris/minSleeps/8 We have used them worldwide including Paris for years. It is a lot cheaper than multiple hotel rooms and are located near attractions. You'll live like Parisians!

With only five days, and assuming that you haven't been to Paris before, I'd stay right there. Plenty to do and see. With eight people you might want to split up sometimes and do different things. Versailles and Giverny are both easier day trips than Normandy or Champagne if you really want to get out of town.

I'd suggest an apartment if you can find one big enough for all. More room to spread out, save time and money with breakfasts and maybe some dinners "in," easy to do laundry, easier for different ages to keep different sleeping/waking hours. Goog'e "Paris vacation rentals" for companies that offer apartments or put you in touch with owners. It may be safer to work with one that offers apartments directly because there's reportedly a "crackdown" on short-term rentals by owners of individual places. See other posts on this board.

You do not HAVE to reserve a TGV train in advance except possibly for very busy holiday travel times. It's much cheaper to book in advance, though.

Here's Rick on that:
"...many countries have at least one category of high-speed train that always requires reservations — most notably France...."
I’d recommend booking as far ahead as possible for the following trains: TGV (France’s high-speed trains)
http://www.ricksteves.com/travel-tips/transportation/trains/reservations

TGV's require reservations, but it is almost always possible (except, as I said, at extremely busy holiday times) to buy a reservation with your train ticket right up to the moment the train departs. "Reservation" does not automatically mean advance booking, just that you get assigned a specific seat when you buy the ticket.