There are way too many things to do with children. To give a real good answer, a little bit more information would be useful, e.g. what are the main interests for the kids? Boys or girls? How old are your in-laws? Are you driving by car or by train?

Expect looooong lines and a rather small castle in Neuschwanstein. There are more - and by far less crowded - castles in Bavaria, e.g. Schloß Herrenchiemsee. You could also consider Legoland, the Salzbergwerk Berchtesgaden or the Deutsche Museum in Munich. Pls. keep in mind that the drive from Oberammergau to Nürnberg is more in the 3-hour region - and even more on heavy traffic. Getting around is easy and does not require German - just a good map. Best if you buy them in one of the many gas stations or book shops.