First and probably only family European trip

Well, for the kids an obvious choice would be Paris...then on to Normandy. Paris would offer the shopping, desserts, Louvre, Eiffel Tower, etc. as well as a day trip to Disney. On to Bayeux and a trip to Normandy. You could also work in a night on Mont St. Michel (looks eerie in the fog... maybe ghosts?) Depending on exactly how many days you might be able to work in a flight to Munich, rent a car, see some of the south of Germany (castles, etc) and drive to Salzburg. (I have taken a few pre-teen and teens there and they loved it)

If your son wants to see Omaha Beach and/or Normandy, he might really like to see Dachau, outside Munich. We found the site to be in very good taste and someplace everyone needs to see. We took our 11 year old there, and it and Anne Frank Haus were the highlights of her trip. Salzburg's another great city, and the hills are truly alive.
Tell your daughter that bakeries in Europe, and especially Germany, have such yummy looking treats. But they don't taste as near as good as they look.

You have 14 days (or less with travel and jet lag). Spending a few days with friends leaves what - 10 days? Dip into Switzerland to see some mountains for 2 days. That leaves 8. Spend a few days in Paris (for shopping and Disney, I guess - you'll have to shield their eyes from a lot of museums and churches, however), then take a couple days to see Normandy - maybe three days and spend one on Mont St Michel, which is kind of cool without being too museumy. That's your trip. There isn't really time to go anywhere else unless you don't go to one of the destinations I just listed. Good luck.

As James said, there is no German Chocolate cake in Germany. There are very delicious cakes and desserts here though. Poor David has gone to the wrong pastry shops. There are factory produced bakery products sold in chains and there are hand made cakes in small bakeries. The difference in taste and quality between these is astounding. Most ice cream stores in Germany are owned by Italians, so lots of gelato to be had. Most of them also sell spagetti eis. Your kids will love it. If you visit Munich, go on a bike tour. The kids will enjoy it and so will you. With just 14 days, you may need to trim your plans down a bit. Perhaps set up a priority list, start figuring out the time it will take you to get to each of your wished for cities, and then how many days you would like to stay there.

A 15-year-old's need to shop and eat dessert can likely be satisfied nearly anywhere, and I don't know what to say about tracking down ghosts, so it might be best to pick destinations based on the parents' and son's interests, which seem more Europe-specific, and indulge your daughter's interests wherever you end up. If you finish up a 2-3 day Swiss adventure in Zürich, Salzburg is a 5-hour direct train ride on the Railjet (figure about 130€ or so for the whole family if you buy at the Austrian railways website.) Salzburg is a fun place. Take the train to nearby Hohenwerfen to tour Hohenwerfen Fortress and see the free-flight falconry show. Munich and Dachau are only a couple of hours by train from Salzburg; travel there on a BAYERN TICKET (a train daypass for Bavaria, 30€ for your family.) Half way between Salzburg and Munich is the town of Prien, where you can visit Herrenchiemsee Palace on an island in Chiemsee Lake. You should plan to have a car for the Normandy beaches. If you go to Paris (and I'm not sure that's imperative if museums and churches are out!) rent a car on the outskirts (like Versailles) since driving within Paris stinks (or catch a train to Caen and rent the car there.)

Would Black Forest Cake qualify as German chocolate cake?

I think German chocolate cake and Black Forest are different. German chocolate is more coconuty. But Black Forest cake (Schwarzwaelder Kirschtorte) is awesome! Sometimes it can be a bit potent, though... You will be very close to the Black Forest.

I know you are looking for help planning your trip, and this is unsolicited advice, but may I suggest you pick up a copy of the book "Take Your Kids To Europe". Its a great book that will help you get over the "teen and tween" hurdles when trying to plan things to do. The book has Great suggestions so that Everyone has an amazing trip.

Just curious, do you have 14 days total, or 14 days after having spent time with your friends in Thoiry?

Don't sell your kids short ... I am not a huge museum person but get kids to churches and some sort of museum - give them a chance! We did a two hour tour of the Louvre on a Wednesday night through Paris-walks and it was perfect for my son (then 8) as we saw the highlights. If you go to Salzburg take the bus to Berchesgaden and visit the documentation center and Hitlers Eagles Nest. The best part about traveling sitting outside your comfort zone and exploring places totally different. NO reason to go to Disney! Remember you must book a family room - not like here where you can have 4 in a regular hotel room.

Linda,
We are planning on taking an overnight flight so we get to London in the am. so I guess we will have 13 days,

A Sound of Music tour would be idea for you (not necessarily for your teen). I haven't done one because I'm not a SOM fanatic, but I've heard the bus rides between stops are singalongs - everyone's belting out "the hills are alive...". There are evening walks that are ghost oriented (Edinburgh comes to mind, but they don't have a patent on them).

"A Sound of Music tour would be idea for you (not necessarily for your teen). I haven't done one because I'm not a SOM fanatic, but I've heard the bus rides between stops are singalongs - everyone's belting out "the hills are alive..."." That depends on your family and your familiarity with the movie. My 24 year old daughter would love to do the SOM tour, both now and when she was a teen. But we enjoy the movie. I thought about doing a tour the last time we were in Salzburg, but we were with my German relatives who had never even heard of the movie. Maybe next time... Does your now 13 days include the day you go home? If it does, you only really have 12 days. And it is difficult to count your arrival day as a day, by the time you land, get out of the airport and get to the first place you are staying. Not to mention jet lag. So maybe 11 days. You definitely don't have time for 5 countries.

If you are planning on flying into London, then I would think London, Normandy, Paris, Thoiry/Geneva would work well. You might be able to add one more city, but that's spreading yourself pretty thin. Where do you want to fly home from? The catacombs in Paris would be fun for someone that likes ghosts and hauntings. They are pretty creepy. And climbing to the tower of the Notre Dame and visiting the gargoyles, up close and personal, might also be fun. Happy planning!