Arriving in MUC staying 24 hrs

Note when you say trains and the train station, I think you really mean the metro/subway system for commuting (referred to as S-bahn and U-bahn) which has stops all over the city. The main railway station (Hauptbahnhof) would be for longer distance trains.

Take a look at Hotel am Viktualienmarkt. We have stayed there several times. The location is great as it is a block from the big market there, but also a short walk from the metro. Lots of restaurants in the area. The hotel has a lift, and they do have quad rooms that would fit you two and the kids, and then the other people can get another room.

It might be a mistake to take on Munich with the elderly. You'll be spending 1.5 hours just getting from the airport to Munich central station and back. Then you'll be walking to somewhere more pleasant for lodging and perhaps again for your evening meal through the big city.

How about a smaller town? I would suggest the attractive and cozy town of Freising, which is just 10 minutes from MUC airport by train. The Achat Hotel Corbin, the Mercure, and the Bayerischer Hof in Freising are not far from the station. Or you could get a short taxi ride from the airport to any hotel you like. You might arrange breakfast at your hotel. The smallish old-town zone has plenty of restaurants. The Weihenstephaner Brewery is there, and I think there's a Weingarten in the old town.

You can board the S-Bahn at the airport, either S1 or S8, both go to the main station (Hauptbahnhof), but they also go through the heart of Munich and stop in Marienplatz and Karlsplatz. Near one of those three stops should be a good choice of hotels, restaurants, beer halls, and things to see.

You really do not need the app or to plan a train, this is mass transit, simply head to the S-Bahn station at the airport, and hop on the first train leaving, they leave every 5-10 minutes.

I mostly agree with Russ about staying in Freising. I've stayed there twice when I had an early flight out of MUC in the morning. It's a rather nice place to stay, and you're just a short bus ride from the airport in the morning. The first stay I had dinner at the Parkcafe, on the Fürstdamm in Freising. The second visit I went up to the Weihenstephan brewery (the oldest still operating brewery in the world) and had a pleasant dinner under the Linden trees in their Biergarten.

The 12 minute train (actually a regional train to Regensburg) from the airport to Freising only runs every hour. Between those trains, the S-Bahn to from the airport to Freising has a change at Neufahrn and take 33 minutes, but there is a 16 minute bus from the airport to Freising Bhf. I'd take the bus.

As an alternative, there are several places to stay in Neufahrn, where the S-Bahn to Freising changes. It's only an 8 minute ride from the airport to the station if you stay there, and 8 minutes back to the airport in the morning. After you check in, if you want to go into Munich, it's about a half hour from there to Marienplatz, the center of Munich.

As you leave Neufahrn, there are some more stops, each with accommodations (Eching, Unterschleißheim, Oberschleißheim, Moosach), on the S-Bahn line, each one farther from the airport but closer to downtown Munich.

We like to stay in Freising our last night before flying out of MUC. We like the Bayerischer Hof that Russ mentioned. We take a taxi from there to the Airport as we are forced to fly out so early. The hotel can arrange the right sized taxi for your group.

Freising is a nice small town with scenic waterways wandering through the town. It has the Weihenstephan Brewery mentioned and the Baroque Domberg (Cathedral on the Hill) for sights. There are multiple cafes

We enjoy the Wirthaus Huber for a pleasant meal.

https://weissbraeu-huber.de/en/