We did a spring break trip. Probably 11 or 12 days. Took the kids out of school for 2 days at the front end. It doesn't matter in middle school and elementary school.
My 13 and 10 year olds liked Rome and Venice better than Florence.
Venice is cool just because of the location.
Take a rowing lession with Venice on Board. Go to Murano and see glass blowing.
Rome is cool...even if you don't go inside anything. There are mosaic-making experiences in addition to the gladiator school.
In Florence, my kids liked what was more of a kids' museum with replica DaVinci inventions. They also liked a day tour to see the Leaning Tower of Pisa.
https://www.ciaoflorence.it/en/tour-detail/13
And they liked our hotel.
https://www.hoteltorreguelfa.com/
We flew into Venice and out of Rome. We used trains only.
I have been to Pompeii as a kid. I think it's worth it for kids to visit.
Try AirBnB Experiences to research tours your kids might like.