Hi! Our family with two kids (7yo and 10yo) will be visiting Munich (day trip to Nuremberg), Salzburg (2D1N) in Dec. We are taking trains; not planning to drive.
Some questions:

- If we had time for only one trip, Neuschwanstein + Hohenschwangau or Herrenchiemsee Palace + boat?

- I had originally planned to do a day trip to Salzburg and 2D1N to Heidelberg. But after more research, it seems Salzburg might have more to explore than Heidelberg, especially since we would be travelling over the Christmas period. Even though shops/attractions are closed, I get the impression that Salzburg won’t be as quiet as Heidelberg. Would you agree?

- Should I then put Heidelberg as a day trip from Munich?

- Did anyone get Christmas market fatigue? Too much of a good thing?

- I’d like to visit Berlin too. But that would mean travelling from Salzburg to Berlin on a 6-7h train. We’ve done 14h flights but I’m not sure how my kids would take to such a long train ride. Please advise.

Thank you!

All depends on the total time you will have at your disposal. Don't forget that in December days are short and it can get very cold.
Not the best time for a boat trip on Chiemsee.

Cold, short days.

I like Berlin, but best in the summer

basically as neckervd said

Market Fatigue. Yes I get bored after a couple of hours tops, while sugar and booze to survive also limit the fun

I get the impression that Salzburg won’t be as quiet as Heidelberg. Would you agree?

I can't compare Salzburg to Heidelberg specifically, but in my experience Salzburg has more open during the holidays than say, Munich. For that very reason, I've spent 2-3 Christmases there. Expect Salzburg to shut down around 2 pm on Christmas Eve, and partially come to life again on Christmas Day.

You've not mentioned how long this trip is in total, so hard to say if you'll get Christmas Market burn out or not. Also hard to say what will and won't be open. Do you have some specific dates?

I've found this website invaluable when planning trips to Salzburg:

https://www.salzburg.info/en

If you're visiting in December, it’s best to keep your itinerary relaxed. Munich and Salzburg are great choices for the festive season, as both offer beautiful Christmas markets and a cozy winter atmosphere. Boat trips may not be ideal due to cold weather and short winter days. Including Berlin could make the trip hectic due to long train travel. Also, visiting too many Christmas markets may get boring, so it’s better to choose just a few. If you're traveling from the UK, remember to apply for a Germany Schengen visa from UK early and prepare a clear itinerary.

If Op is British/US/Aus?NZ they do not need a Schengen visa. Whether or not they need an ETIAS by then remains to be seen, but if they do it will be easier to get than a visa.
If however they are a nationality that requires a visa they need to have everything in place and provide all the documentation requested well in advance it is true.