Agree with Chani and Laurel...Start in Venice. Some things to consider, Venice is not friendly to
wandering around with 2 yr. olds. Are you taking a stroller? 10 years ago we had my 3 year
old granddaughter there. We had a light weight stroller and while it worked fine, it did take
us longer to navigate, lots of cobbles and bridges. We picked up the front and back to cross the bridges.
You may want to look carefully at these sites for info.
http://europeforvisitors.com/venice/articles/venice_airport_boat.htm lots of critical information here.
This may be your best option for getting from the airport into the city.
While its more expensive it will save you an immense amount of time and frustration.
http://www.venicelink.com/index.php?page=shop.product_details&flypage=000flypage_escursioni.tpl&product_id=54&category_id=10&option=com_virtuemart&Itemid=43&lang=english&vmcchk=1&Itemid=43
If you're confident travelers consider renting apartments. with a family these will give you
a lot more room and most likely be less expensive than a quad hotel room. here is one in Venice
that meets your needs, is centrally located and easily reached by water taxi.
http://www.cross-pollinate.com/venice/apartment/3458/Ca-Morea.
They also have apartments in Florence and Rome. Great to work with.
For Tuscany check out this agriturismo. Centrally located and things for the kids.
You will need a car. Rent it in Florence.
http://www.poderivalverde.it/farmhouse-chianti-1.html.
In Florence and Rome take a taxi from the train station to your lodging.
You may want to use the TripAdvisor Cities app to your phone. Once loaded it works offline.
Hope some of this helps. Have a great trip.