2 1/2 weeks in Italy - traveling with little children

Hello - My fancy cousin will be getting married in late July in Castello di Casole. We are trying to plan a family trip around this event. That being said, trying to figure out the best itinerary traveling with a 3 and 5 year old - that is interesting, relaxing, and doesn't require too much time on the road. We will have about 2 1/2 weeks.

We are thinking of flying into Pisa
then CT
Then - Florence?
Siena and surrounding towns
back to Pisa?

I would appreciate feedback on the best way to organize. And recommendations on nights in each place. Want to book our CT accommodations soon! Also - train or car?

Thanks in advance!
Have fun!

I would definitely recommend you check out accommodations on agriturismo! Staying on a "farm" would afford you room for the little ones to enjoy the grounds and space. Many of these accomodations offer pools which would come in very handy with your little ones.