The one place that comes to mind is:
We have stayed there and it is a lovely old house turned into a little hotel. In the house itself are 6 double rooms each with own bath and 6 single/double rooms with shared bath. (This might work for the children.) They have a very cute dining area for breakfast and that might work perhaps for evening gatherings. The trick would be to find a time-frame in the summer where you could book out the entire hotel for your reunion. The hotel is right next to a quaint little biergarten (one of our favorites)which would be great for evening meals. And it is also right next to the lake in the Englischergarten (Munich's Central Park) and a 5 minute walk to the famous Seehaus biergarten, which is a must during your visit, and great for a large family gathering. There are many possibilities for family outings in the park - great for the kids to feed the ducks, take paddle boat rides in the summer and great for morning runs, evening strolls or daytime bike rides.
The hotel also owns an apartment building next door where they have 20 apartments(we have stayed there too). These rooms have mini kitchenettes, which might be nice for some of the family members to make meals for young fussy eaters or just to have a bit of milk, bread, etc. in the fridge, but it's not in the house itself and it is a more 70's style building, not as cute and charming as the house. They also own and manage the Hotel Biederstein which is only a 5 min walk away, a cute boutique type hotel. See their website here: http://www.hotelbiederstein.de/html/rooms__prices.html
All 3 properties are about a 10 min walk to the U-bahn station U3/U6 Münchener Freiheit and from there you can be in the city center in no time at all.
When I spoke to the receptionist this morning, she suggested for a large family gathering and the possibility to book out the entire hotel you should talk directly to the manager - her name is: Roselinde Zankl, she speaks english, and her contact info is on the website. As you would be booking for a large group it wouldn't hurt to see if you can get a bit of a discounted rate as well. She also mentioned that if you weren't able to book the entire hotel (say due to availability for your dates) there would be many possibilities between the 3 properties I mentioned above to accomodate 15-20 guests and they are all more or less next to each other. Just be prepared that these are European size rooms and bathrooms, which will be on the smallish size for what you might be used to in the USA. But that is pretty much what you will find here! PM if you want any more information.