I've always found Immigration (pass port check) to go quickly in Munich. It's on the same floor as your gate, at the middle of the concourse (assuming you're using the old part of T2). When you exit Immigration you go down elevators to the luggage carousel (whether you have checked luggage or not). When I've flown into MUC, I've always gone through Immigration and gotten to luggage pickup before the luggage arrived (I carryon, but the girl seated next to me on the plane was still at the carousel waiting for her luggage when I exited Customs). I'm just saying that your biggest delay on arriving will probably be getting checked luggage.
There is also a bank ATM in the room with the carousel.
If you know what you are doing, it takes only a minute to get your ticket from the machine on the S-bahn plataform, but if you buy the Bayern-Ticket online, it's valid for the entire day, so the airline would have to mess up hugely for you to lose the ticket.
One thing you should know - take the S8 to München Ost. The departure board shows only the S1 as going to Ost, but that's confusing. Ost is the S1's final destination. It goes around to the west side of town first, to the Hbf, and then through the main S-Bahn tunnel under town to Ost, and it takes longer to get there than does the S8, which doesn't show Ost as a destination. The S8 will show another town west of Munich (probably Herrsching or Weßling) as it's final destination, but it goes early to München Ost, then through the tunnel and on to it's final destination.
1 person of the adult group (parent or grandparent) can be accompanied
by an unlimited number of own children/grandchildren aged under 15 for free.
The group with unlimited children/grandchildren of the first adult can also include a second adult, related or not, for example, the second parent, a child over 14, or just a friend. Or the group can include up to five people of any age. But the group cannot include unlimited children and more than two adults, or two adults and all the children of either adult. Someone tried this once with both parents, multiple children, and one over fourteen child. The conductor made them buy a second full fare ticket for the older child. He could have fined them. So one adult with all of their under 15 children is 25€; Add a second adult, and it's 31€ with all of the first person's children.
we can't get a Bayern ticket until after 9am on Saturdays
You can buy a Bayern-Ticket anytime in advance, but it will be dated and only valid on the date shown. When you use a ticket automat, it will ask you if you want it for today, tomorrow, or another day. If you select "another day", it will show you a calendar to select the validity date. On weekend days, you can use it at any time. On weekdays, you can buy it at any time, but you can only use it after 9 AM. You can start travel before 9, but you must have full fare ticket coverage for any travel before 9. Sometimes a train from Munich Hbf will leave a few minutes before 9. Look at the schedule for that train and see the first station at which it stops after 9, then purchase tickets to cover your travel to that station. Once you get to that station it will be after 9, and you can use the Bayern-Ticket.