Been to Italy lots of times! Crusieship excursions are very pricey and limiting especially for two children.
Driving is always our choice from Florence port. Road are good and marked well. If you have never been to Florence need to go into the city.
Car rental company very close to port is Libert Rent email:firstname.lastname@example.org
You cannot drive in Florence so will need to get a map with parking spots and park for the day. There is a nice place close to Pitti Palace. Get a ticket for a tour towards end of day. Walk up towards Duomo crossing the Ponte Vecchio. Do the Duomo. We like the Santa Crocce not far from the Duomo (leather stalls behind church) then head to the Oiazza della Signoria for pizza and Gelato. Then back accross Ponte Vecchio to Pitti for your tour.
You can stop at Pisa for a quick look at the tower.
Rome is not a driving city!!!!! Again if the kids have never been to Rome..A MUST!!!!!
The train station is about a mile or so from the port an very inexpensive option to getting to Rome, it drops you at a good stop in Rome. You get get tickets to a hop and go bus right at the Train Station http://www.rometoolkit.com/wha..._hop_on_tour_bus.htm
You will want to take the kids to Pompeii. You have to hire a guide in the ruins, if you get a guide at the port to take you there you will have to hire another inside the ruins. Just so you know, you will have to pay for two guides.
Any other questions feel free to email me.