We stayed here with our two young kids (almost 2 and 6 years old) for 5 days. The place is very beautiful, in an authentic apartment building. You have to climb a few steep stairs (a bit of a hassle with a stroller), but it's worth it ;) The place has a small but adequate bathroom, a nice kitchen and living and 2 good bedrooms. It is a bit noisy, but not too much.
The apartment is perfectly situated at walking distance from all the great sights in Florence's center, but in a relatively quiet street. No need for a car. Taxi from airport takes you there in <30 minutes. Great restaurants at an arm's length (e.g.Francesco Vini), a nice playground at ~10 mins walk (Piazza Massimo d'Azeglio).
Sara & Massimo were very helpful and flexible in setting up a baby bed, in arrival/departure times etc. Communication in English is possible through Sara.
We warmly recommend this place to - among others - cityhoppers and families with young kids!