Beware....most places are closed on Sundays. So basically, you can walk around the city, go to the Englischtier Garten (largest park in Europe I believe, it has a couple of great beir gartens that even the kids might enjoy, plus authentic German food and drink of course. Most the people form the city will be there weather permitting.
Try to take Mike's bike tour. Kids will love it and you get a great overview of the city.
Have fun, I love visiting Munich, I just hate that almost everything is closed on Sundays.