Family of 5 visiting Germany this summer

From one base you can only explore so much territory on daytrips. I'd suggest two. Near Frankfurt, a base on the Rhine and a second base in northern Bavaria would make sense for a mix of small-town and cities. 5 days in each is about right. From either base you can reach FRA in 1.5 hrs. or less. St. Goar is a very good Rhine base for train travelers. Daypasses in the area allow for cheap and impromptu daytrips. Sights and activities: St. Goar itself and Rheinfels Castle Braubach with Marksburg Castle Bacharach Oberwesel Linz Cochem (Mosel River) with Reichsburg Castle Burg Eltz (castle near Cochem) Remagen's WW II museum Apartments: Rhine View apt., Other apts. This daypass for families is a great deal. See map for coverage area. KD river cruise info. (you get a 20% discount on fares with the train daypass, but check out family pricing too.) Northern Bavaria: Würzburg (city) or smaller towns like Iphofen, Neustadt-an-der-Aisch, or Bad Windsheim are well-located on train lines for outings to... Bamberg Würzburg Rothenburg Nuremberg
Bad Windsheim's Franconian Open-air Museum

I would also suggest two locations. The ages of your kids are great for seeing all the wonderful castles that Germany has to offer. I have taken a few of my grandkids to Germany and a few favorites are Heidelberg, Rothenberg (although we stay in Dinkelsbuhl) and of course Nueschwanstein Castle. I am sure Jo will be able to give you plenty of specific advice. But, if you can manage just one move to the south I think you will find its worth it. Another favorite place in Germany (don't know about logistics on train... I always drive) is Lake of Constance.

I understand your wanting to setup a base camp. But 10 days is relatively short time for exploring an area with so much to see. I agree with the others about splitting your time in two places. Your time would be more efficient took a train to your base location(s) and rented a car for day trips. You'll see twice three times as much by auto.

To add to Martin's list: Fritzlar, Darmstadt, Wiesbaden, Miltenburg, Heppenheim (and all of the Bergstraße), the Deutsche Weinstraße, Königstein, the Odenwald, Hunsruck, Vogelsberg and Spessart mountains, Bad Homburg, the Neckar River Valley, Bad Wimpfen, Buztbach, Bad Nauheim, Alsfeld, Kronberg im Taunus, Eppstein, Eltville am Rhein, Hessenpark, Saalburg, and inumerable castles, all worth visiting and all less than an hour away from Frankfurt by car. If your visit falls between 14-23 June, don't miss the week-long Hessentag festival, which will be held in Kassel this year.

I agree that 2-3 places would be better. To find an apartment (fewo, short for ferienwohnung), go to the town's website - usually www.(town name).de. Look under the TOURISMUS tab and it should be obvious where to get the list of fewos. Some websites have english versions, although I have found several that have abbreviated listings on the English versions. Use a translator if necessary. If you determine some towns to stay in, re-post and get some specific recommendations. I always recommend Bavaria Ben's non-commercial website - www.bensbauernhof.com for very useful trip reports and other practical information. I have never taken a car in Germany (although have used a bicycle twice) and have used the train on my last two trips. Depending on what you want to see, train travel may be sufficient. Using the various train bargains may be a big money saver. Kids travel free on some of the tickets. See the Bahn.de site - find out about * Länder-Tickets * Schönes-Wochenende-Ticket
* Quer-durchs-Land-Ticket

From Frankfurt you can see these places (all less than 2h away from FF/M):
Marburg, Limburg an der Lahn, Fulda, Eisenach (Wartburg castle), Wetzlar, Büdingen, Idstein, the Rhine valley, Heidelberg, Worms, Mainz, Aschaffenburg, Würzburg, Speyer Most of these towns offer perfectly preserved centres, some of them great churches or palaces, there are many castles, parts of the region are also very picturesque. There is much more to see than you can do in just 10 days, that's why I would not recommend two bases. You would just waste a day and make you holiday more stressful.