Have 1.5 days to visit Tuscany. What is the best way to get around with a 6 and 2 yr old?

Rent a car. See Siena, Volterra and San Gimignano as they're out in the same direction--south and southwest of Florence a short distance. Many stay out in the agriturisimos in the countryside. There are also many great wine towns in the region.

With only 1 and a half days, you really don't have the time to "travel around" Tuscany. You could do a day trip to Siena by train - it's about 1 1/2 hours each way. I think a tour would be tough since you wouldn't have any flexibility with the pace - and flexibility is always key when traveling with kids!

Rent a car and follow suggested route above.

Rental car gives you to most freedom to travel at your own speed. It doesn’t take long to get to the countryside from Florence. Any the countryside is beautiful. Agree, an agriturismo would be very nice. If you do opt for a train out of Florence to another city, I would choose Lucca over Pisa. It’s much more beautiful, and the walled city is less congested than Pisa.

I'll vote with Siena by bus or Lucca by train That way you can still sightsee at your own pace but without the hassle of carseats and parking and all that. I would not book a tour: do some reading in advance and just wing it on your own. Oh, and you could also do 1/2 day in Pisa by train if one of the aforementioned towns - which I'd give FULL days to with little ones in tow - don't appeal but I think I'd do either of them before choosing Pisa. Also, be flexible and choose dry, pleasant days for your excursions once you get there: no need to book train or bus tickets in advance.

Unless the tours were private, they probably would not accept children that young.

It kind of depends on what time of year it was, but I think I would go by train or bus to Siena and spend the whole day there. Any time that's not the busy tourist season would be best. Our grandchildren are 5.75 and 2 now, and what they would like to do is to run around the campo in Siena (it's like a park but it's paved and people just sit on the pavement like it's grass and kids run around with soccer balls and balloons and pinwheels and it's absolutely lovely) and also to see the floor of the cathedral there (more animals and some strange but interesting scenes). There's also a museum that was a hospital across the piazza that has frescoes that would be enjoyed by kids. And they could visit at least one of the shops that sell souvenirs of the various contrade of the town --- the neighborhoods have adopted mostly animal mascots (like caterpillar and giraffe and porcupine) and you will see these on knick-knacks and flags and pottery and sculptures and street signs everywhere.

Just saying all this makes me want to take them back to Italy and go to Siena right now! The older one was 3 turning 4 when we were in Italy with him for 2.5 weeks and he loved it and still talks about it.

Walks of Italy apparently does allow small children as this is in their FAQs

If you are traveling with a small child and stroller, please let us know before booking as we will need to reserve space on the bus for both.<<

They have an all day Tuscany tour that might fit your needs- not sure what your budget is though!
https://www.walksofitaly.com/florence-tours/tuscany-day-trips-from-florence/

Includes a winery visit so you would not have to worry about having a designated driver if you intend to taste.

I'll echo kathy's recommendation of Lucca. It's an hour 19 by train. On your way back to Florence, if the kids are not tired, I can recommend Pistoia. Cute Midieval town midway between
Lucca and Florence. They frequently have market fairs during the summer.

For a wine tour, this one is just a half day and very reasonable. Price for your whole family
about 140€.

https://www.getyourguide.com/florence-l32/chianti-wine-tasting-half-day-tour-from-florence-t4433/

Trains are definitely the way for your family to travel to the towns I've listed. For Siena, the bus
Is the preferred mode of transportation. It drops you off in town whereas the train stop is downhill from the center.

With children do young you need the flexibility of a car. No dragging luggage off and on trains
while holding onto your little kids.

Where else will you be on this trip (before and after the 4 days in Florence) and how were you planning to get around from place to place?

Unless I read it incorrectly, I got the feeling from the OP that they were not intending on overnights from Florence; that they were just going to day-trip from there so hauling luggage around on train or bus isn't an issue. ????

The car could be a little more of a challenge if having to park outside of the town centers. - as there are restrictions around driving into many of them - and covering the extra distance on foot with the tots. Public transport will generally dump you out closer to the middle of the action. :O)

Our kids are a little older but I think trains are the simplest travel options. No need for car seats, and your full attention during travel can be on scenery and enjoying time with the kids-not navigating and parking. Also tours not aimed at kids can be long and boring. We love to download Rick's audio guides and self-tour. Focus on 2 sites per day in town, and picnic in a park for lunch. Let them run around in piazzas and bribe them with gelato!
I haven't visited Tuscany yet but we've spent 2 weeks in Italy; our youngest was 9 the first trip. But possibly day trip to Siena for sure.

we had a rental car in Tuscany as we were staying out in the countryside. Getting to the rental car in Florence on our way out of town from Santa Maria Novella train station was a huge pain - the cobbled side walks were not stroller/luggage friendly. Plus you check in at the office, then walk 2 more blocks to get to the garage. Parking was a pain in most of the Tuscan towns, though the Rick Steves book had very good suggestions for parking lots. We rented a car seat - it was fine, a little well used, but served the purpose for the week.

That said, It is much easier to reach the hill towns with a rental car. Siena is possible to reach via slow train, but I don't know that you can get to San Gimignano via train - you'd have to take the Florence/Siena train to Poggibonsi then a bus. So either your choice of stops will decide the method, or your transit method will determine which stops are feasible.

Our 3 year old loved Siena and San Gimignano, she loved running around Piazza del Campo in Siena (someone else mentioned above) and up and down the Duomo steps in San Gimignano. She also loved the gelato stops in each town of course :) The drive from San Gimignano to Volterra is amazing, not that the 3 year old cared though.

Florence and its busy-ness were overwhelming for us as a day trip. We definitely overdid it with travel times and day trips - it's so hard to minimize! If I were doing it again, I would do less daytripping and spend more time strolling in one place and not moving around so much.

In Florence, we went up to the Boboli Gardens and had fun running our daughter around.

Siena is possible to reach via slow train

As Gerri mentioned earlier, bus is usually the recommendation for traveling to Siena from Florence as it drops off in the center of town instead of outside of it.

https://community.ricksteves.com/travel-forum/italy/bus-from-florence-to-siena-563ddbc6-a4d5-4f96-aa2a-d7b13cfef3db

Have you considered hiring a driver? We're going to Siena in May and I do not want the hassle of a rental car so I am hiring Paola at http://takemeoutintuscany.com/ for two days. So far, she has been very responsive to my emails and is customizing itineraries for us. Her rate ranges from 350 to 450 euros a day depending on what you want to do.

At this point, it would really help if the OP would get back to us with feedback on any of the options offered up?

Not knowing how much you plan on packing into each day with a 6 and 2 year old, and requirements for naps, my suggestion is a little on the conservative side. Fiesole. It's a rustic hilltop town that overlooks Florence. Nice and close to home incase of meltdowns. Good luck and enjoy!