Stephen,
From the information posted above, it looks like you have already purchased tickets on August 4 for a flexpreis ticket for three from Munich Hbf to Innsbruck via Mittenwald on August 17 for 102.90 EUR. Is that correct?
If so, a taxi from the airport with a quick tour of central Munich is going to cost about 100 EUR. From taxifarefinder it is about 65 EUR just for a straight ride from the airport to Munich Hauptbahnhof, and it will vary according to traffic. So I am adding on 50% for the driver's extra time for a half hour tour.
The alternative is to take the S-Bahn into Munich as Lee suggested. You say you don't like subways, but the Munich S-Bahn is more like a regular train except it travels underground in central Munich. All the cars are at platform level, so no climbing up and down stairs to board or get off. The main stations like Marienplatz and Hauptbahnhof have both elevators and escalators in the stations, so again, no stairs to contend with. You would need to buy an all-network group day ticket for 23.20 EUR (money saving plan to follow). So if you are landing at 9:10, there is a good chance to be on the 10:24 S-Bahn to Munich. That will get you to Marienplatz at 11:02, take the elevator to street level and take in the view in front of the Neues Rathaus (City Hall). There are side walk cafes fronting the square to relax and have a coffee and a treat. At noon, the Glockenspiel in the Rathaus tower will perform. Expect a large gathering in the square just beforehand to watch the show. Immediately afterward, take the elevator or escalator back down to the S-Bahn station and take the next train going 2 stops to the Hauptbahnhof. Trains go every 2 to 3 minutes that time of day and it is only about a 3 minute ride. At the Hauptbahnhof, again elevators or escalators to the upstairs station. Quickest way is to exit in the direction the train is going. Gleis 27 (track 27) is to the right and is set back a short distance from the main tracks. Your train is at 12:32 and you should be there early to get a choice of seats.
Now, money saving idea. If you did indeed already purchase your tickets as above, you can get them refunded on-line for a 17.50 EUR fee. So you will get back 85.40 EUR on your credit card. Now wait until you arrive in Munich and purchase a 3-person Werdenfels ticket costing 27 EUR, that will be good for the ride to Marienplatz and then the short ride to the Hauptbahnhof and the train as for as Mittenwald. At the same time buy 3 tickets from Mittenwald to Innsbruck for a total of 31.50 EUR. So now you will have spent 17.50 plus 27 plus 31.50 for a total of 76 EUR. The other way would be 102.90 for the ticket to Innsbruck plus 23.20 for the ticket from the airport to Hautbahnhof for a total of 126.10 EUR. This saves you over 50 EUR, which will buy a very nice lunch for the 3 of you in Garmisch or Mittenwald. You might want to use the airport station ticket window for peace of mind, in which case that will cost you another 2 EUR.
@ Lee Do you concur Doctor?
Here is a detailed map of the Hauptbahnhof:
http://www.bahnhof.de/file/bahnhof-de/6493602/WPaFA2SexkeoxoU1VjKgBQEue2k/6511088/data/Muenchen_Hbf_de_PDF.pdf