I believe that the only 27€ ticket for a group of 3 adults from Munich to Garmisch is the Werdenfels-Ticket, which is like a Bayern-Ticket for a smaller area. It is not for specific trains. Rather it is for any desired regional trains in 2nd class for that entire day on certain routes, after 9 AM on workdays.
The Werdenfels ticket cost 19€ for the first (main) traveler and 4€ for each addition traveler. I'm not certain, but after reading the Conditions of Carriage, I think that if you take advantage of the "children free" with a Werdenfels-Ticket, you might be limited to the main adult and one additional co-traveler (not counting the children), but that's not what the Bahn ticket website says. Ask about that at a ticket counter.
The only train specific tickets I know of are the Savings Fare tickets, and those tickets must include at least one segment on a train of the Bahn (ICE/IC/EC/IR). Most trains on that line from Munich to Garmisch-Partenkirchen are regional trains. And Saving Fare tickets for 3 adults (children are free) start at 39€ one way.
The website for the Bayern-Ticket indicates that you can get up to a 20% discount on entrance fee for museums, zoos, and Bergbahnen with a Bayern-Ticket valid that day. There used to be a list online of all the discounts, and I believe it included the Zugspitzbahn, but I can no longer find the list. You might ask at a ticket counter if they have a list. It's possible that a Bayern-Ticket for your party for 31€ (instead of a Werdenfels-Ticket) could save you more than the 4 extra euro on Zugsptitzbahn tickets.
Update: The website for the Bayern regional rail (Bahnland Bayern) says that children under 6 are free and uncounted with the Bayern-Ticket regardless of whether your use the Familien-Kinder option, or not.