in pratice, two options...
Ciao O'Reilley Family,
I've been really struggling with my suggestions to your questions - because of the number in the group, transportation is going to be key in where and what you end up doing.
Given that you really only need a car/van for those 7 days in Tuscany, let's consider that you're also on vacation and want to make things as simple as possible. While it is easy to reach most major towns by train, smaller towns and the countryside in Tuscany require a car, or as you've considered, at least a taxi or two to get to town to the train station and then use trains.
Let's look at car/van rental: the largest vans available on car rental sites are for 9 people and space for luggage is limited. Some examples of the models marked as for 9 people:
Fiat Scudo, Fiat Ducato, Ford Transit minibus, Mercedes Vito, Citroen Jumpy and the VW Caravelle.
For example, on
Rentalcars.com, I found the MERCEDES VITO 2.2 MINIBUS 115DCI (for 9) with automatic transmission for around 520 euro for 7 days in October picking up from the Rome Fiumicino office (7 random days as you didn't give your dates -- otherwise, you could take the train to Florence and pick up here). Similar searches from same departure/return point also showed vans on Alamo and on Budget but rates seemed higher. Since the majority of your group are children (but not sure of their ages and therefore size), you might be able to all fit in the 9 person van? So would a large van and travel expenses (tolls and gas) be more than if you had two cars? I am not sure, since cars also get more mileage. Consider both options and see which one might be better. You can get an idea of toll costs on
Autostrade - gas currently is around 1.50 euro per liter, a record at the moment. Hopefully it will come down soon, the average is generally between 1.20-1.30 euro per liter.
On the other hand, combining car rental with train travel is a good alternative. Children under 4 travel free with accompanying adult but don't have a right to their own seat. Children from 4 to 12 travel at 50% off and get their own seat (need to show ID to show their age). There is also a "family" ticket for families of up to 5 people - the children still get the 50% off but the accompanying adults also get 20% off their tickets. Minimum ticket cost is 10 euro so this fare is only convenient when tickets would cost more - when buying your tickets, make sure to ask the vendor first if the family fare is better for your trip. Make sure to buy tickets from your starting location to the final location regardless of the need for transfers in between, that way you the appropriate fares and discounts where applicable. I wouldn't depend completely on train travel, though.
So taking into consideration these points, these are my suggestions to make your vacation simpler (even if not the cheapest):
1. STAY AT A VILLA, WITH CAR/VAN RENTAL: stay at a villa in the countryside and rent a large van for 9 people or two cars and use that to get around during your week in Tuscany - you wouldn't have to worry about getting into the nearest town with taxis and then taking trains. Although you can definitely plan some day trips with use of car + train. For example, to visit Florence and not deal with driving/parking in the city so you drive to a local train station close to where you are staying and take the train in... or you can drive in and park in Piazzale Michelangelo for free and then walk down into the city - great views from the piazza overlooking the city. It is just 15 minutes walk down to the center.
This in any case will make your choices of villa and location more open - I wouldn't say you should confine your search to below Siena and Arezzo/Anghiari. A more central location for all day trips would be the Chianti area, anywhere between Siena and Florence and even further west near San Gimignano. This would place you closer to Pisa and the coast.
Here you can find lots of choices:
Villas in Tuscany...
On this list, in particular I'll point out as good matches:
the
Cabbiavoli villa
the entire
Villa Vivai at Fattoria il Lago
Villa Piaggia
either
Casa in Pietra or the Casa il Pozzo at Residence le Coloniche
the
Villa Vianci
2. STAY WITHIN A CITY AND USE TRAINS: staying within the city/village would make train travel much easier to use for all of your trips. Of course, the difficult part is to find the right villa for your group. There are many right on the outskirts of Florence - many are set up as B&Bs nowadays but many are also divided into apartments which can be joined to give you the whole villa for a group. Many villas and homes in the countryside (often called farm houses because they are more rustic but essentially the same as a villa) are now divided into apartments -- if you were to rent 2 apartments for 5 people each, even if there are more apartments in the same property, you almost always have your own garden area, separate entrance to the apartment, parking space and just the use of the pool is common (usually covered by October though).
For example, this
villa can be rented as a whole. Located on the outskirts of Florence, a few minutes walk from the villa you can catch the Florence ATAF city bus #14 into downtown and from here visit Florence as well as catch trains to Pisa, Siena, Arezzo, Lucca, etc.
This
B&B near Pratolino is 5 minutes from the ATAF #25 bus stop that brings you into Florence in about half an hour. You could see if the whole B&B is available and get it for your families.
You really have a lot of options with this second possibility, being in the countryside but using the city's public transportation that reaches into the outskirts. You'd just have to definitely make sure you are close enough into Florence (or another town) to ride in and then use the train system for day trips.
I'd avoid being out in the middle of nowhere in this case where you'd need taxis to get into the closest town - those can be hard to obtain, particularly for larger groups. Many don't know that Florence is so small, that as soon as you go over the hills that look into Florence you are in the countryside - you don't need to go much south beyond Piazzale Michelangelo and you're already surrounded by olive groves and countryside. Or north, just behind Fiesole and all is countryside. (Another example: this
winery offers accommodation in apartments, you could get two, and is so close to Galluzzo and Florence that calling a taxi from here should not be a problem).
Start looking at the places I've mentioned, getting a better idea of what you'd like to do and go from there.... maybe search for your accommodation first, what you'd best like and making it fit with either option #1 or #2 above.