12 day trip to Europe with 14 people, seven of which are aged 7-15

First, traveling with kids means that if you visit museums, they tend to want to leave after 30 minutes. Still, kids and learn the history and culture, but need a bit more. I traveled with mine and when young they loved things like going to the top of the Eiffel Tower, taking a gondola ride in Venice and climbing the dome of St. Peter's in Rome. My kids absolutely were in awe of the Sistine Chapel.

As for 12 days doing three cities, is possible, but you still have to travel between the cities.
I suggest that you research what you wish to see at each city and determine if the number of day allotted at each city is enough.
All three are wonderful places, but going to Munich, you almost need to do day trips to Garmish-Fussen to see the castles and Zugspitze (highest mountain in Germany). Also, Salzburg, Austria is special and you should plan some time there. A day would not be enough.

Vienna, were were there last Summer and did a concert that was great. Not sure your kids would?

Another, more expensive option is to consider taking a Danube River cruise. We took a great one from Regensburg, Germany to Budapest. Fantastic, consider Gate 1 Travel, that was the company we used. Viking and other lines have them, but they are always more expensive.

If you go to Munich and want to see the concentration camp of Dachau, I don't recommend that you take your pre-teens there.

Thank you for your thoughts!

I would say Paris is a natural "next place." It has a lot to offer young ones: the combo of Giverny and the Orangerie, the Montparnasse Tower (I prefer it to Eiffel), and especially all the gorgeous parks and gardens. Train connections in any direction for your second base. Maybe Bruges? My 12-year-old enjoyed our day trip to the coast, Atlantic Wall Museum and DeHaan. A little further along the coast is an aquarium. You could stay at the coast and do a day trip to Bruges instead.

Rhine valley, Alsace ( Strasbourg and environs, journey into Black Forest )

Thank you for your ideas!

Paris and Amsterdam?

Not on your list, but a week or longer in the Dordogne has a variety of things for all ages. We enjoyed kayaking on the river, prehistoric caves, exploring medieval castles, great markets, great places to bike. We were seven, renting a house for ten days in June 2023. Our then 9 and 12 year old grandkids loved every minute. If we had had time, there was a museum or two, a cathedral, a chocolatier, an apple orchard, a goose farm, beautiful gardens...lots of places to visit in one of the most beautiful areas of France. We have to return to see everything we missed. And LOTS of "les plus beaux villages de France". We rented a large place (slept 8 with 3 bathrooms) through Gites-de-france. They have larger places, and sometimes 2 or 3 houses/apartments in one place.
Just a thought for your large and varied group. You will need 3 cars or two vans for 14 people, and this area is best by car. (Not much available in public transport.)

On your mentioned options, I loved Munich and Vienna, and feel they offer the biggest variety for your varied age group. If you go to Munich, the Deutsches Museum is full of interesting scientific displays, both ancient and modern, plus the Residence Museum, especially the Treasury, is full of beautiful objects like "St. George". The older folks can tour a brewery. Watching the glockenspiel in Marienplatz is fun for everyone. There are fun markets and parks for picnics. Check the Germany forum for more ideas. We were there for five days and could have spent longer, especially with a few days trips. (I recommend Dachau...sobering but important)

Vienna has the nearby mountains, the Lippizanner horses, the Opera house, Schonnbrun Palace and the Vienna Woods, Grinzig for music and white wine, the giant ferris wheel in Prater Park, the gorgeous art museum with wonderful Breugel paintings...I always think of Vienna as a city for celebrating art, music and life!

Prague is interesting, but I don't see it as having the same amount of appeal for your varied age group. I am glad I went when I did, but don't have a desire to revisit it, as I do have for Munich and Vienna.
My two cents! Bon Voyage!

I would seriously look at staying around Lake Constance. You have dinosaurs, zeppelin rides, boating, castles, huge public gardens (Mainau), museums, the Rhine Falls, cable cars up into the Alps, hiking trails, local fests, antique shops, flea markets, fresh produce, etc. Downtown Konstanz is very nice, but the smaller towns around the lake are great for exploration. There's a reason this is a popular destination for families.

Maybe two locations within the same country or nearby that offers City time & beach or lake time, since you will be there in June. I wish you luck.

Safe Travels.