With kids these places might be interesting:
The Saalburg, a reconstructed Roman fort near Bad Homburg.
http://www.saalburgmuseum.de/english/home_engl.htm
Hessen Park, a large open air museum, also near Bad Homburg.
http://www.hessenpark.de/index.php?id=home
The Antique Tram museum in Frankfurt, where kids can climb around, etc. Grown ups enjoy this one too.
http://www.hsf-ffm.de/index.php?id=64&L=1
For history, consider going to the Judengasse museum, built on the exposed ruins of the Jewish Ghetto, which was occupied from 1462-1810. This is very hands on and all in English. It is located next to the Holocaust Memorial Wall, which if your family is interested in History, will be a site they will remember. Over 12,000 names on this in alphabetical order. They can find Anne Franks name, as well as her sister Margot and their Mother Edith on here
Senckenburg Museum of Natural History
http://www.senckenberg.de/root/index.php?page_id=589
The Communication Museum might also interest them, as well as the Archeology Museum as it has lots of Roman and Celtic artifacts found in Frankfurt.
Go to the neighborhood of Höchst, which has dozens and dozens of half-timbered houses, lining cobblestone streets and all sitting atop the old city wall overlooking the dry moat that surrounds the old schloss. Also located there is the Justinus Church, built in 850, this is one of the finest examples of Carolingen architecture left in Germany. The Höchst Porcelain factory also offers tours.
A visit to the Emperors Hall in the Römer might be of interest. It has oil paintings of all 52 Holy Roman Emperors lining the walls, beginning with Charlemagne in 800. The majority of these Emperors were elected in Frankfurt and the last 10 of them crowned there. Visit the cathedral where this happened.
The Rose and Tealight Festival is happening on that weekend in the Palmengarten.