Just a note to research dates for Xmas markets depending if you will be here during Xmas. Many will close 12/22 due to the holiday falling on a Monday (most close 12/24 or 12/23 in general, few will reopen 12/27 for 3-4 days as a New Years market).

Salt Mines --tour underground and big slides, etc. These are near Salzburg/Berchtesgaden
Ludwig castles:
Palace at Chiemsee
Neuschwanstein ("disney castle)
Linderhof Palace

All are interesting and fun.
6 year old would prefer Neuschwanstein and, probably, the castle in Salzburg


https://www.european-traveler.com/germany/visiting-schloss-linderhof-and-park-bavaria/

Science Museum in Munich

We’ve been in Munich the past two christmases. The markets are obviously lovely. My son is a bit older, but he enjoyed the outdoor ice skating and a bus tour to Neuschwanstein. Our first year, we added a few days of skiing at Stubai glacier, but based out of Innesbruck (just be careful of altitude sickness) - it’s the closest point nearby that is high enough to be snow sure at that time of year.

We are planning two weeks in Germany in Dec 2018, partly to visit family but then to travel around a bit and also head to Salsburg. I would love to hear about fun things for kids that age to do in Dec. we will go to several christmas markets as well. I am looking for anything. We already took her to "tropical island" indoor waterpark and may do that again. we will mainly be in Hanover and further south bavaria I think.