Austria help

You don't mention what you are interested in - which would be a help in providing advice. Why did you pick Austria? and Why in December? What are the boys particularly interested in?

In Vienna I particularly like the Kunsthistoriches Museum. Franz Josef loved sending his minions out to collect whatever they could find and it has a particularly nice Egyptian Collection - as well as lots of other stuff of course.

Vienna and Salzburg will have concerts galore, so that is no problem, other than picking only a few. Neustift in Stubaital is a really lovely valley with lots of winter things to enjoy (I hope - but of course it always depends on the weather), cross-country skiing and sleigh-rides, a trip to the head of the valley and a gondola up to the Glacier - even if you are not a skier. If you are bored there is always Innsbruck. Christmas Markets all over the place. Pick some small ones for a more local flavor.
Austria has lots of excellent local websites to investigate further.:
https://www.wiener-staatsoper.at/en/
https://www.viennaclassic.com/en/specials/salzburg
https://www.stubai.at/en/stubaital/villages-of-the-stubai-valley/neustift/

Are you trying to mix some skiing in with some sightseeing? Just trying to understand the
4 nights in the Stubai region.

I am guessing you are flying into Vienna and that cannot be changed, but you are looking
at a lot of traveling in not that long a time.

Also, you seem to have defined the itinerary and not decided what to see, so why are you
thinking of these places?

Don't want to dampen your excitement, but, with your exact travel schedule, figure out how
much time you have in each place. 2 days in Vienna could be just 1 night, same for Salzburg.
You'll spend more time on trains than on the the ground. Are you spending any time in
Munich or is it just an overnight and departure?

Sorry, but if it were me, I would reexamine this plan. If you haven't bought train tickets yet,
don't, until you review things.