I completely agree that if you want to see specific sites in Rome or Florence, a private tour will allow you to see much more than a
Disney Cruise excursion, and will allow you to adjust the itinerary based on what your family wants to see.
The historical area of Florence is easily walkable, so you could take the Disney Florence on Your Own bus excursion to get to the city, see what you want for a few hours, then take the bus back. My family did that twice. It makes for a long day, but you can sleep on the bus in the afternoon.
In Rome, the top sites are spread out, so having a driver is a help. There are several highly rated companies that will pick you up from and drop you off at the ports. We have had wonderful experiences with Rome in Limo and Joe Banana Tours, but there are others too.
I have not been to Cannes. From Villefranche, we have just taken the train to Nice and to Monaco and walked around, but that would be farther from Cannes.
I also highly recommend spending a few days in Barcelona before or after your cruise — there is a lot to do there.