Hello. I love Germany and think you have made an excellent choice for a country to visit. Many travelers don't realize how big Germany is; here is a map that superimposes Germany over California and Nevada to give you some perspective as to just how big it is.
There are 2 travel philosophies: (1) race from place to place and see everything you can pack in or (2) take a slower pace and experience more of the people/places you visit and less of the roads/trains of those places.
Rick Steves' books fall into philosophy #1. His Germany guide recommends seeing Munich, Bavarian "castles," the Upper Middle Rhine Valley, Rothenburg, "more of Bavaria and Tirol with a side trip to Salzburg," and Berlin in 10 days (essentially Berlin --> Munich --> Upper Middle Rhine Valley). Russ is clearly of philosophy #2, and I will readily admit I am highly-biased in that direction, too (as are most of the regular posters in this forum).
The Rick Steves itinerary would be exhausting to me. In terms of distance and driving times (I checked on Google Maps), it's the equivalent of doing San Francisco --> Los Angeles --> Las Vegas in 10 days (with a stop somewhere in the desert to account for Rothenburg). Anyone who drove Rick's itinerary (like anyone driving the California itinerary) would also likely get a nice introduction to the German word "Stau" and its impact on travel-day plans.
So... you get to decide which travel philosophy suits you and your family, as well as your goals for this trip, the best. If you go with philosophy #1, own it and some us will try to help you as best we can (though expect to hear things like, "You're doing too much!"). If philosophy #2 sounds good, Russ's recommendations are excellent. If Russ's plan sounds great, but there is one place that you are dying to see, let us know, and we'll try to help you figure out how to incorporate it.
One final tip for future trips... think about building your itinerary and then buying your plane tickets (instead of building an itinerary around plane tickets). That lets you figure out what you really want to do, and then figure out how to make the flights work. From a philosophy #2 perspective, your current trip would lend itself well to a dip into Munich/southern Bavaria if you had an open jaw itinerary into Frankfurt and out of Munich; with your current flights, though, it's a lot driving to get back to Frankfurt.
Happy travels! Germany is awesome.
(Sorry this post disappeared and reappeared -- meant to delete something else!)