When you say you've seen the sights of Rome, which of the sites/sights have you seen? You could easily spend your one night in Rome and maximise the time exploring rather than sitting on a train. For example, have you seen the Baths of Caracalla? You can hire a Virtual Reality audio/visual guide at the ticket kiosk. Your kid might enjoy that.
Otherwise you could get on the train to Florence and have a walk around there. But you really have very little time and will lose three hours each way on the train.
Or a day trip from Rome to Ostia?
Thank you for your reply I appreciate - we have had superb local driver for 8 hours our last visit to Rome who drove us all around the city and got us into the major sites, i.e. Colosseum, Parthenon, Vatican, the many fountains, dined, have been to Florence several times as well, love it naturally - what we are planning is arriving in Rome for two nights before cruise and thought take advantage of where we could possibly go either by train or a driver take us there - have heard the Amalfi coast not reasonable - is Positano? will be there end of July - or perhaps Tuscanny?