Frankfurt-region activities for families with kids:
Frankfurt or Opel Zoo (located in a beautiful area of the Taunus mountains overlooking Frankfurt).
The well-preserved town of Büdingen (contains what people travel to Rothenburg odT to see, but without the tour buses and trinket shops).
There's a pretty good castle between Frankfurt and Büdingen that offers falconry demonstrations, but I forget the name.
Hessenpark- Open-air folk museum. It's only half finished, but perhaps worth a visit if you're into that kind of thing.
The Mittelrhein- needs no introduction
Marburg- Beautiful, well preserved university town. The old town wraps around a hill, with a castle and castle garden on top.
If you'll have a car, the small village of Münzerberg north of Frankfurt has a prominent castle ruin that you can see for miles (just drive north on A5 and you'll see it).
Taunus Wunderland: I haven't visited, so I'm not sure if this is more of an amusement park or adventure forest. Either way, it seems to be popular.
If the kids like hiking, consider a stroll up to the summit of Großer Feldberg (the prominent mountain with the TV towers on the top) on a clear day. It's a fairly easy hike and the views from the top are stunning.
Finally, if you want dinner with live Schlager music, head to Rüdesheim am Rhein for dinner some night.