For whatever it's worth, I thought I'd give you a quick rundown of what my kids liked best from our trip to Italy. We just returned last week, spending 11 nights spread out over Rome, Tuscany, Lake Garda and Venice. Our kids are 9, 14 and 16. It was difficult keeping the 9-year-old entertained, but the teenagers enjoyed most things.
In Rome, the Colosseum was definitely the biggest hit. We did a kid-friendly private tour of the Colosseum, the Forum and Palatine Hill with just our family of five. Walking around Rome was also popular, visiting the Spanish Steps and the Trevi Fountain. The Pantheon drew bored yawns from the 9-year-old and I think even to the teenagers it was just another famous old place. We also visited the Vatican and Sistine Chapel with an early entry VIP tour. As adults, we loved it, and the artist in our family, the 14-year-old, loved it, but the 16-year-old and the 9-year-old were kinda bored within an hour.
I will say the Torre Argentina cat sanctuary in the center of ancient Rome was quite popular with all the kids and my husband and I. That was really a highlight. Oh, and our 3-1/2 hour food tour through Eating Italy of the Travestere food district. Wow. Every one of us enjoyed it. Even the youngest kid.
On to Tuscany. The hill towns all started to look a bit alike to the kids. But we still had fun exploring and walking through the towns. And eating. Again, eating was the most popular pastime. And gelato. Sometimes we'd get it twice a day.
Lake Garda was very popular with the kids. Our 16-year-old declared the ferry ride around the lake, visiting towns including Salo and Gardone, as his favorite part of the entire vacation. A beach day at the lake was also a favorite of the two younger kids. The water was warm and the beaches were nice, albeit it pebbly. We rented lounge chairs and umbrellas and ordered lunch from a nearby restaurant. It was lovely and so scenic.
Venice was our 14-year-old daughter's favorite place. Same with our 9-year-old. They both said it was so unlike anywhere else they'd ever been, or anything they'd ever seen, that it was a fantastic experience. We did a private family tour around the city, visiting St. Mark's and other famous sites, ending with an hour water taxi ride around Venice. We also later did a 30-minute gondola ride. The kids loved that, too.
Enjoy your planning!!!! As others have said, Italy in July is hot. Very hot. We basically walked around with a light sheen of sweat all day, every day. But by the second week, we were used to it. Here at home in it was 90 degrees yesterday and my husband declared it a lovely day without realizing how high the temperature was. When I told him, he said, "Oh, I guess I'm just used to much hotter weather after Italy!" Because it's not just Rome that's hot - it's absolutely every city. But hey, being hot in Italy in July is far better than being home in hot weather. It didn't deter from our enjoyment of the trip. Altho my husband and I did say that if we go back a third time, since it was August when we visited last year, we will likely opt for May or late September/early October to experience better temperatures.