First trip to Germany

One more thing....10 day trip. Thanks!

Wow, what lucky kids!! I got $50.

I'd suggest Berlin and Cologne. Maybe Heidelberg too, but maybe not... it's a fairly average sort of place, and there's plenty to see on day trips from Berlin and near Cologne along the Rhine and elsewhere. In fact, I'd consider splitting the Cologne stay - do a 2-night stay in Cologne and also a few days south of there on the Rhine (near Koblenz) where you can do some really nice outings to old-world villages that aren't so easy to find around Berlin or Munich. Braubach and Bacharach are especially nice. Boppard is a nice old town too. It is also this area where you'll find Germany's strongest concentration of medieval castles.

There's nothing wrong with Berlin and Munich as a choice either. But day trips from Munich will typically require more time - and castles and old-world villages tend to be a long distance away.

You have targeted cities large enough that a car does not make sense as a mode of transport between your destinations. It will also be a hassle taking day trips by car from these places, especially from Berlin.

I'm looking right now at an ICE train saver fare of €86 total for four adults between Berlin and Cologne (sample December date) for a direct trip of 4.3 hours. If you love driving you can rent a car in Cologne for outings. But it's less hassle and less expensive to visit the Middle Rhine Valley (castle-country) or other places near Cologne by train; day passes for families are very inexpensive.

Check flights into Berlin and out of Cologne (or Munich) ("open jaws") at the ITA site (which makes searches easy) then purchase elsewhere.

Does that ten days include your travel days? How many nights on the ground?

Personally i would go with Berlin and Munich. Berlin is an awesome city with plenty to see for both old and young nad Munich also has a fair bit to see. The two cities would also give you a glimpse of the diferences within Germany, North and south.
Cologne is nice and the Dom is marvelous, but other than that there is not much to see. The city can be done in a day. Heidelberg is also small enough to be seen in a day trip, so the Berlin/Cologne/Heidelberg option would entail a lot of travel with not so much to see.

Fabulous. I was leaning toward that itinerary. 8 days in country. So, Berlin to Cologne by train. Don't have a passion to rent a car. Want authentic Germany...don't need the Neu..stein experience. Will check into Koblenz. Would love to do a short Rhine river cruise. What about Bacharach or Beilstein?

I would skip Koblenz as a base town in favor of a smaller more atmospheric village. Try Braubach (small) or Boppard (which offers the best assortment of accommodations and dining options of all the villages and IMO is more authentically German and less touristy than the other towns.) Beilstein is a tiny postage-stamp place on the Mosel without a train station - attractive but not much to do there. From Boppard you could be in larger Koblenz by train in about 15 minutes if you needed some "big-city" services. Doing a day trip up the Mosel to Cochem or Burg Eltz Castle (hike there from Moselkern station) is easy with a start in Boppard. It's also a good place to end a river cruise (Bingen-Boppard.)

Only "8 days in country." OH. In that case I would absolutely skip Heidelberg. Schedule 4 nights for Berlin (so that you have 3 full days there.) Leave Berlin early on Day 5 for Koblenz area for 3 nights (Days 5-7) Then Day 8 and night in Cologne and fly out from there.

I agree that you need at least 4 nights in Berlin, then maybe 1 night in Cologne, then 3 nights in Rhine area and maybe fly home from Frankfurt. Of course that depends on where your daughters want to go, if this is a graduation trip for them they need to have some input. Maybe they would rather do Berlin and Munich, which at that age I would have preferred (if you're talking high school grads). The trip will be more fun for them if they help plan it.

Agree with Berlin and Munich. Both have a lot that would interest all age groups. The Victualienmarkt (you can eat all types of sausages here), eating at the Haufbrauhaus, visiting the English Garden, and the Glockenspiel in Munich (it performed at 11am when we were there) are not to be missed. But which ever towns you pick you're sure to enjoy them!

"Agree with Berlin and Munich. Both have a lot that would interest all age groups."

I think that's true of the Berlin/Rhine option as well.

Rhine:
WW II museum in Remagen
Summer bobsled, near St. Goarshausen
Rheinfels Castle
Doll and Bear museum
Wine-museum and tasting ( St. Goarshausen, perfectly legal for older teens!)
Open-air museum, Bad Sobernheim
Burg Eltz
Falconry Exhibition, Reichsburg Castle in Cochem
Boppard's chairlift to Gedeonseck
Gedeonseck terrace (short walk from top of chairlift)
Rheinburgenweg (Rhine Castle Trail)
Marksburg Castle (Braubach)