To get to the Westin Excelsior from Rome Fiumicino airport, take the Leonardo Express train to Stazione Termini.
Before boarding the train in the airport, make sure to validate the ticket in the yellow machine. (on a subsequent train to Pisa, we failed to validate, and were fined, along with most others on the train, 5 Euro). At Roma Termini, go downstairs into the subway and take the A subway to Barberini station. From Barberini, you can either take bus 95, 63, 62 or 116 along Via Veneto, or take a pleasant 5 minute walk along Via Veneto. You will see Excelsior on the right hand side right after the US Embassy.
The train information
http://www.trenitalia.com/en/treni_s...express02.html
The interactive Roma Metro information
http://www.metroroma.it/MetroRoma/HT...Disabled=false
We did this with our 6 and 9 year-olds without difficulty. The metro is very conveniently located. With luggage, it may be somewhat of a trial as the sidewalks are cobblestoned in sections, and very narrow. The Barberini station is the second stop up the line from Termini.
Good luck, and enjoy Rome!