HOw old are the kids? I went as a teen and then subsequently as an adult so I am not very au fait with stuff for younger kids.
The Pergammon museum is stunning - I remember being over-awed even as a shallow teen.
I think that the House at CHeckpoint CHarlie is still there - it was a fab museum, very cluttered but infinitely fascinating. It may or may not have been poshed up.
The Jewish Museum and Memorial are unmissable, imo but tough.
The only castle is really the Charlottenburg but its more a palace than a castley-castle. For those you need to journey down the Rhine.
The Reichstag dome is a very popular tourist thing. As a kid, I remember liking walking round the Food Hall in the KDW (Kaufhaus Des Westens - posh dept store).
I'd be tempted to do a cycle tour - its a flat city so easy to get around and some the big spaces are a long walk. There is also quite a lot of interesting modern architecture so getting around on a bike is a good way to see it all.
DH & I watched some midnight cabaret but prob not for kids!
Its an amazingly vibrant place - I love it. I am happy just wandering around finding markets and beer gardens and mad art galleries. I hope you have a fab time.
I went as a teen and remember Potsdam as a highlight. We were on a coach but I would think you can get train or bus out there. We saw San Souci and the Cecilienhof, the place where the Postdam conference (at the end of WW2) took place. I would love to go back.
I was there in April its a fantastic city and I'm sure he will get to practice his German as people didn't speak much englis! The riechstag is great but you have to book well in advance We did a tour of the bunkers from ww2 would be great for kids The Jewish museum fantastic building and very touching exhibit Not really anything at checkpoint Charlie The wall exhibition is good We also went to a Bavarian themed restaurant which kids would love the waiting staff wore all the gear I can't remember then name but if you are interested I'll find out the details. You will have a great time
The natural history museum is very good - the archaeopteryx, lots of dinosaurs. The Pergammon is amazing - check for a late night opening. One of the museums you need to book an entry for - can't recall which one, think it might be New Museum. Odd stuff happening on the lawns at Unter den Linden (main road to Brandenburg gate) Segway tour of the city (expensive, tho) Beware of bicycles. Currywurst. Some of the old churches, etc are interesing. One building showing the history of the city, with complicated staircases, showing a lot of the damage done during the bombing. I found the people there very friendly, and most under 30 speak excellent English. DO book a visit to the reichstag building. The views are fascinating, with audio guides explaining what the various buildings are. I think children get in free to a lot of places. There is an excellent online tourist information service. Look into getting a 3 day underground pass - possibly one with discounted entry to some places. Enjoy!