mathemilu,
You've received lots of good comments from the Germany experts on the forum, but I have one comment......
"a way to do both Germany and Italy, my girlfriend is not very found of that idea. We've got a talk about all this at the dinner and she told me that she would find it exhausting to take all those flights to rush to many places in the same month"
If you're going to be in southern Germany, it's not a huge problem to reach northern Italy via the Brenner Pass, so no flights would be necessary. It's a very relaxing trip by train so not exhausting at all (you can sleep during the trip to make it less exhausting). However, as you appear to want to focus on Germany this time, you can certainly visit Italy on another occasion.
Are you using Germany guidebooks to plan your trip? Those would be the best way to find sights that would most interest you and your family. The books also provide lot of good information on the opening & closing times of various Museums, admission charges, transit information, hotels, restaurants, etc.
A few additional thoughts on the most recent Itinerary you posted....
- Berlin - 4 nights is good, but with that time frame I'd suggest skipping the day trip to Potsdam. There are LOTS of sights to see in Berlin. Keep in mind that you may be jet lagged for a few days and with small children you may not be able to move as quickly and will have to take more frequent rest breaks.
- Dresden - 2 nights is fine, but that will only provide about 1.5 days for actual touring. If there are a few sights you want to see there, you could take add one night there.
- Nürnberg - 4 nights should be good, and I'll leave the question on which place to visit on a day trip to the collective wisdom of the forum. Rothenburg ob der Tauber is very scenic and the children might enjoy the Käthe Wohlfahrt Christmas store. Getting to Rothenburg from Nürnberg will require at least one train change (Steinach as I recall). At that time of year, it may be crowded during the day time.
- Salzburg - 2 nights would be good. While you could visit Salzburg as a day trip from Munich, you might enjoy the city more by spending a few nights there. The town has a different character in the evenings.
- Munich - 4 nights is about right. You could also take a day trip to Neuschwanstein Castle in Füssen (will take about half a day). You can do that trip on your own, or take an organized day trip with Radius Tours - your choice.
- Mosel - Rhine - 7-8 nights might be a bit too long, but again I'll leave that to the collective wisdom of the group. You might allow one or two nights in Luxembourg prior to your flight home. Which town in the Mosel-Rhine area were you planning to stay in? There's been some discussion on that topic on the forum recently, and some seem to prefer Boppard. I find St. Goar too "busy" and that will likely also be the case when you're there. I stayed in Bacharach when I was last in that area. I'm sure the group will have some comments on that.
For planning your rail journeys, the best resource is the Bahn.de website. Don't be too concerned about not being able to speak German. While it's always nice to learn a few of the polite words (ie: Good morning, Good Evening, Please, Thank you), most of the people you'll be dealing with will be able to function in English. Europeans are very clever with languages, so you may also find some that can speak French (although it will likely not be Québécois).
Good luck with your planning!