OK, you really don't have much time, but you want to keep the girls happy and make them want to travel as they get older. The biggest thing you can do is build that desire. You can easily spend the whole time in Munich. I've spent months there and haven't seen the whole city. It's a little short on castles, but there's so much other stuff...
Plan at least half a day at Nyphenberg Place. It's a German Versailles; amazing buildings, fantastic gardens (which should be in fall colors when you're there), and great art. It's also a "real" palace, where the nobles from all over the world met and waited on Royalty's whims.
If the girls like art show them the Alte and Neue Pinokotecs (the Old and New Art Museums). There are half a dozen museums within the same area (Egypt, etc) so if they're having a good time they can run from one to another. If they like science or seeing how things are built hit the Deutsches Museum on it's island in the middle of the river. Enough museums...
Dachau...let me start by saying it's important, and I think everyone should see it. But it's seriously depressing and can give you nightmares. I've been several times (once on my own, the other times just showing friends around Munich), and It's usually good for drinking afterwards. I'd pass this trip.
Castles - There are several right in and around Munich, but they're more the Schloss type (large defensive houses) and probably not what your girls are thinking of. If you had more time I'd make a couple other suggestions, but since you don't here's what I think the girls will like best; go to Nuremberg for a day. It has a great castle, it's a really pretty old city, it's got the old walls, open air market, and good food they will like. I've laid out an easy tour in a previous thread: https://community.ricksteves.com/travel-forum/germany/organized-tours-of-nuremburg and I think it would work for you. Plus it's a simple train ride to and from, no need for anything other transport. Don't go to Fussen, it's crowded and seriously over-rated.
(On an aside, if Munich is too expensive the day you fly in, consider flying into Nuremberg and out of Munich. Castles and city one day, stay overnight, and train to Munich in the morning. If you want to do this I've recommended hotels in Nuremberg before also.)
Shopping: The main walk thru old Munich is basically off limits to driving, and it's a major shopping zone. So is much of the old part of town. In Nuremberg the best shopping is along the river down to the Trodelmarkt (an island in the river with shops). Most shops close at 6pm. Walking around and window shopping is good at night.
The Victual Market (in Munich) is a series of open shops that specialize in food, and it's the best place to grab and go (or to sit outside if the weather's good). Plan to eat breakfast (which may not be what the girls expect) at the hotel. Eat at an Imbiss (snack stand) during the day. (Germans eat their big meal at noon, so restaurants are crowded then). You really should hit at least one beer garden just so the girls can say they've done that. Hofbrau Haus is the most famous, but the food's not that good. Try the Lowenbrau or Augustiner places; they serve stuff other than beer. The one in the English garden is good too. Remember the menu is posted outside, so you can pick and choose. (Let the girls pick a couple small places so they can see something a little different; hole in the walls can be really good.) Try to find a jazz cellar one evening. If the girls like music they can be fun. if the weather's good there's usually street musicians out and around the main tourist areas. Munich has a major reputation for musical students.
It's going to rain while you're there unless you're incredibly lucky. Plan accordingly. Don't overpack. remind the girls you're only going for a week. Last, Munich is pretty easy to navigate if you use the Marienplatz and churches as landmarks. But a good map on the smartphone is helpful.
