We just returned from 4 nights at Sassi Bianchi, family of 4. There are a few farmhouses, we were there pretty much alone (beginning of the season still).
First we were at the Ginestra farmhouse - it was ok but a bit "too" rustic (cracked mirrors, old furniture...). The heating stopped working on the second day so we got moved to Lilium, the largest and nicest of the villas. It was huge (our bedroom looked like a king's bedroom, huge, gorgeous views, canopy bed...).
Overall we enjoyed our stay, although the place is a bit isolated, especially in the off season, at the end of a winding road (though only 10 minutes from San Gimignano). Concetta and Paolo were very nice and helpful. Keep in mind heating costs a lot (we paid over 100 Euros). There is a nice patank area.
By the way a tip in general about Tuscany - we enjoyed the "out of the way" places more then, for example San Gimignano which is very toursity. Try Montalcino, a beautiful, sleepy little place with a great fortress for the kids or Monteriggioni which has a nice little armor museum.More