We are on the same cruise as you. If you can change plans to come into FCO a day early, do so! Other than that, just a few random thoughts. You arrive at 8am, by the time you colect bags, clear customs, and catch the leonardo Expresss to Termini it will be 09:30. Drop bags at the "left luggage" in termini, buy tickets for Civitavecchia, and then walk across the street and catch a hop on-hop off bus. You want to be back to Termini by 13:45 at the latest. You need to be at the ship by 15:00.
Of course you could always hire a cab and do it that way for about 500-600 euros for the 4 of you.
Specific questions, send email to my username at bellsouth.net.
Radiance of the Seas - Nov 2006
Legend of the Seas - Jun 2007
Costa Fortuna - Mar 2008
Legend of the Seas - Oct 2008
Celebrity Constellation - July 2009
MSC Orchestra - Jan 2009
Navigator of the Seas - March 2009
MSC Poesia - November 2009
Jewel of the Seas - June 2010