Rome-Florence-Venice by Train

Use the official Trenitalia or Italotreno websites to book for train tickets.

https://www.trenitalia.com/en.html

https://www.italotreno.it/en

This website has helpful information about the different types of trains in Italy and how to buy tickets.

https://www.seat61.com/train-travel-in-italy.htm

On the trenitalia website - which I have used many times - there may be a limit re. how many tickets you can book at one time. You may only be able to book 4 or 6.... I don't remember what the top limit is, but if you have to buy them in blocks, be sure that a grownup is in each block, because you will also be choosing your specific seats and separate bookings may not offer blocks of tickets particularly close together, or even in the same train car. You will want to have a grownup/responsible person with each group (that is, assuming your grandkids are youngish). The fast Trenitalia trains are called "freccia" and have several ranks, but they are all comfortable and require advance purchase and seat assignments. The trip Rome to Florence will take approx. 3 hours, and only makes 2-3 stops between Rome and Florence. The freccia train from Florence to Venice is a bit faster and makes only a few stops also. You can download a very user friendly Trenitalia app. With a group of that many people wanting to make sure they are all on the same train and able to sit together in groups, I'd make the reservations at least a couple of weeks ahead, although you should also be fine with making reservations 5-15 days ahead unless you want to travel on a holiday. If you make the reservations on the Trenitalia website, you can print the confirmations out at home and bring with you. If you make the reservations on the app it will give you a code to show the conductor when they come through to check tickets.

How far ahead should we buy train tickets?

You don't have to buy train tickets in advance, but since you are a group of ten you can save hundreds getting the discounted tickets asap.

As an example, Trenitalia's cheapest tickets must be bought not less than 2 weeks before the day of travel. The full walk-up fares, called BASE, can be bought up to 15 minutes before the departure time. The difference on the Rome-Florence Trenitalia train is € 30.10 per person.

If you are travelling in 2024 before the 9th of June, note that the tickets of high speed trains will be gradually released before the 9th of December 2023. On the 10th of the December the new schedules for the following 180 days will be fully uploaded, checking before is a waste of time.

If you are using the trains right after the 9th of June, Subscribe to Italotreno's company newsletter. This way you will receive their discount codes first and you will know when the summer tickets are on sale.

And does anyone have tips as to how to book the trains?

Go on the 2 official sites linked above, compare prices, read carefully all the Terms and Conditions of each fare and hit the buy button. Same as buying any other ticket.

I used https://www.trenitalia.com/en.html for these tickets and I usually buy first/business class and reserve seats as well.

Just wanted to add that you are total grandparent goals!!! How amazing for these kids!

You have some good lead time to investigate all possibilities but Trenitalia offers some discounts for groups of 10 or more:
https://www.lefrecce.it/Channels.Website.WEB/?referrer=www.google.com#/group-creation

They also have significant discounts for family groups:
https://www.trenitalia.com/en/offers/frecciafamily.html

Hopefully even though children in the family group would travel free with 1 adult, you’d be required to have/buy a seat reservation so that all in your large group of 12 would have an assigned seat.

Read up, you seem to have options and may just need for tickets to be released depending on your travel dates.