Itineracy Suggestions

Not a bad itinerary, however, If you could made room for doing The Romantic Road, which stretches from Wurzberg to Fussen.

https://www.romanticroadgermany.com

You are already going to Fussen. Rothenburg ob der Tauber is a must see if you visit Germany. It is a perfectly preserved medieval town. Much of Germany was severely bombed during WWII. Cities like Frankfurt, Hamburg and Berlin were almost completely bombed out. What is there not is mainly new. Not so with Rothenburg. Also, other cities and towns on the Romantic Road are preserved, like Dinkelsbühl and Nordlingen. Augsburg is still worth a half day just to see the City Hall and The Fuggeri. Nordlingen was built on the site of a meteorite strike and the city walls were built on the edge of the crater.

You can see the rest of the road (you are already doing the Fussen area) in two full days.

I suppose that 8-year olds don't want to go hiking in the Black Forest. But if you were doing that, you might prefer to stay outside Freiberg, in order to get the free KONUS transit cards that allow you to train to and from the ends of the hike without having the car stuck in the wrong place, for free. There are also nice views from the train, although not as easy to stop and see a waterfall, for example.

I think Frankfurt is greatly overrated. I know that Washington DC does not have real skyscrapers, but surely your sons have seen them elsewhere? Frankfurt is not really worth your time with 8-year olds. Have you researched open-air museums with historic buildings instead? (No personal experience.) If you are in Frankfurt with a car, they might enjoy a half-timbered town, and/or the reconstructed Roman fort at Saalburg-depending on their interests. Since you are going to the Rhine, I would consider Cologne (a far more enjoyable city than Frankfurt) or the castles that can be visited (see our host's Germany tips, menu top left) on the Rhine, rather than banal Frankfurt. I have often found that a two-segment flight to start out an open-jaw vacation in Germany is cheaper than the ground transportation to return back for a RT flight, and the TIME saved is invaluable.

You might want to make sure you can make your 3-hour check in at Munich from Fussen the same day. There is plenty of traffic in Germany, speed limits or not. I also suggest you use the search box for "Middle Rhine" to note that you will be using the (cheap, fast) train to take a one-way boat ride on the river, despite having a car. I wonder if the Mosel (say, Traben Trarbach and Bernkastle-Kues) might be a pastoral-enough substitute for the Black Forest, without all the driving. In some ways, Trier is nicer than Freiburg. (My ideas are related to reducing driving, more than saying you need different attractions, however ... .... You know what your boys will like.)

Just an FYI: your kids will flip over a stay at the Hostel in Bacharach! Family rooms (at least some) are in the turret and the grounds themselves are an adventure--for young and old!

The Berlin Zoo has the most species from any zoo in the world! Kids always like zoos don't they? Anyway we enjoyed it there.

Failing to understand the comment that Frankfurt is greatly overrated, when most of the time is extremely underrated. With many posters believing there is nothing historical or authentically old here, that the city was destroyed, when the complete opposite is true.

A trip to either the top of the Main Tower which offers stunning views of the city as well as the huge city forest, or if you feel like climbing, walk to the top of the Kaiserdom - 323 steps. Cool view of the new Alt Stadt that just got finished.

I would add those extra days to Berlin.

Ms. Jo, I respect your love for your city. I'm a native of New York City. But I have been to Frankfurt twice, for a total of ten days. Compared to the rest of Germany, where I have spent at least 8 weeks in total, Frankfurt is overrated. That is not a "fact" it is an "opinion." (And the airport, as well as the Middle Rhine, have a lot to do with why so many posters here stop in Frankfurt.) Newsboards are as much about sharing opinions as useful facts. On my first visit, I stayed at the CP Plaza (now renamed) in town, on my second visit, we stayed in Kronberg and used the S-Bahn, because we were also making rental car trips.

Thanks for the suggestions so far. Regarding Frankfurt, I guess a rating of a city would depend on your interests and given that going to the top of tall buildings is one of our things in cities we visit, a stopover in Frankfurt makes sense to us since it’s not about seeing skyscrapers in general, but visiting the tallest ones in Germany. Ms. Jo, thanks for the specifics as we’ll be sure to climb it. I guess we can leave our bags at the train station in storage?
My kids do enjoy hiking (we try to do a couple a month at home and try to get it in on vacations that allow for it), and thanks for the heads up about the train/trail options in the Black Forest.
I’m thinking I’ll add one more night to Berlin and the other to Fussen for a daytrip up the Romantic Road. Thanks again.

Yes, you can leave your luggage in the lockers at the main train station. Large lockers cost 5€ and hold quite a lot. You will need coins for these.
https://www.maintower.de/en/

In Berlin, the zoo is very famous, but I enjoyed the aquarium (next door) even more. I only went because my mother wanted to go, but we both had a great time, and didn't even get to see all of it.

Whether one loves or hates Frankfurt, no one can say it is "overrated," since it is usually bashed by everyone. I liked it, but I can see why others don't (I also like Milan and Madrid, two other big European cities that seems to rub a lot of tourists the wrong way).

See the Cathedral in Cologne if you can. Hamburg isn't near anything, but kids and adults would love Miniatur Wunderland if you can fit it in. Near Fussen there are luge runs that can be done in the summer. Also check into the Treetop Walkway, and riverside park and trail by the Nature Center.