Flying into Amsterdam, out of Berlin in Dec. Should we visit someplace in-between?

6 full days is not much for these both cities alone. In this case I would not add another stop although Goslar with Weihnachtswald Market is in between and a great idea. But it takes little time to reach and get away from it - same travel time like to Berlin. If you depart early from Amsterdam you can spend max. the afternoon there and leave early next morning to Berlin.

Link to Berlin Christmas Markets.

If you only have 7 days (and I actually count 6 full days excluding your arrival date and your departure date), you have enough time to really immerse yourself in either Amsterdam or Berlin, or to give yourselves a decent but brief introduction to both cities. Trying to fit in a third destination, even if it is en route between the two cities, is a very bad idea. Keep in mind that you will also lose at least a half day moving from Amsterdam to Berlin, whether you fly or take the train.

Stick to those two cities. You will easily be able to fill your time there.

"...we really only have about 7 full days to see things...My husband is much more of an outdoorsy, non-city person than my girls who want to hit the Christmas markets."

"Things" is pretty vague. You will be using Schiphol Airport (A'dam) and Berlin-Brandenburg Airport. But these airports are entry and exit points on the periphery of these cities and do not require you to "see things" in either place.

• If your goal is to "immerse" yourselves in A'dam and and Berlin, as Lane suggests, then A'dam + Berlin is all you have time for.

• If your goal is to hit some Christmas markets, or to visit Goslar or other smaller towns/cities in northern Germany , you can easily do that over 7 days without ever setting foot within A'dam or Berlin. There would be many options for you in between these two airports. Christmas markets are everywhere. A couple options...

Osnabrück (Christmas Market) is on the A'dam > Berlin train route.
Hanover is too. See Christmas Market reviews here.
Goslar is a stunningly pretty old-world town with UNESCO World Heritage status. If the photo is any indication, it's fabulous at Christmastime too.

Maybe you would have time for one or the other (Berlin or A'dam) plus a couple of other places in between. Just depends on what your vision for this trip is.

3 nights in Amsterdam and 4 nights in Berlin are barely enough for either city, and will leave out the many (not particularly "rural") daytrips from each (like Delft, Potsdam, and Leiden, really major places) that are possible. It is unrealistic to imagine visiting remote places in Germany as well as actually "seeing" two major cities of the world.

You have not really stated if you are willing to change hotels every night in order to visit two or three places a day. That's an unpleasant pace to me, and barely scratches the surface of even a "stopover" place. It was not unreasonable to choose two places that are 450 miles apart, but you need a (personal, not from me) statement of what you actually want to do with the 7 nights. You clearly have a lot of "stakeholders" in your decision-making, but you have not made the realistic decisions that will fail to satisfy everyone.

Just to pick one example, I'm a big fan of Cologne, which is right on a free and independent American SUV trip from Amsterdam to Berlin. It has a lot more than the Cathedral to see, not to mention almost adjacent Dusseldorf. But you don't have 2 nights to fully see Cologne. And a car is a liability while visiting a city. (Cologne happens to be famous for being very walkable from the HBF.) I've spent a week in Berlin, and (more than one trip) more than a week in Amsterdam, with daytrips too.

Disclosure: I'm not interested in Christmas markets, so I can't say if, say, big-city Christmas markets are somehow inferior to those in a smaller city.