If you are counting arrival day and departure day that leaves you just 6 days to explore and don't forget jet lag. Try to stay outside and active on arrival day. Rather than dividing these 6 days up in 2 cities, I would recommend you choose just one city (and don't forget the time it would take to leave the hotel, travel to new destination, since probably by air you must include the time to get to and from the airports, then you have to get to your new hotel and check in.) That will probably use up one of your days. As to which city, I've been to both several times and enjoy them both. In the summer though Rome can be really hot. I remember a June day at the Colosseum that was over 90 degrees. I suggest the family get a good guidebook from the library and explore the major sites in both cities. I would recommend Rick's books as they always include a 2 page chart of major sights at a glance with short description & his star rating.