brian,
I'd also recommend Orvieto, for a number of reasons. It's a wonderful smaller hill town, the Duomo is beautiful (especially the ceiling frescoes on the room at the end), and the children might enjoy having a look at an ancient engineering marvel, St. Patrick's Well.
As someone else mentioned, you could also take a day trip to Civita di Bagnoregio, which might be kind of an "adventure" for all of you (be sure you check the timing of the last Bus to Orvieto, as it's a LONG way back!).
Siena would also be a good choice. It's a bit larger than Orvieto, and would be a bit closer to Florence. If you choose Siena, you might consider taking the Bus to Florence, as it departs from the centre of town (as opposed to the bottom of the hill where the rail station is located), the travel time is not much different, and it stops in Florence right next to Firenze S.M.N. rail station (very convenient location).
Happy travels!