We just got back from a family trip in Italy that included 7 nights in Venice and 4 nights in Rome. Four adults and two kids, 6 and 9. So, I am going to imagine four more people that include a baby/toddler and a preschooler.
You’ve already gotten some great advice here, especially from Chris.
The week in Venice was by far the most successful part of our trip. The most successful thing, kid-wise, that we did in Rome was to go to Ostia Antica. The kids did enjoy Castel Sant’Angelo, but it was extremely crowded in early January. If I were you, I would choose only Venice and Rome, with a day trip to Orvieto and Ostia Antica from Rome. We adore Florence, but your time is so short and your group is so large.
I really, really think you need two or possibly three apartments so that you have floor space for the younger kids, some common areas, couches that are not being used as sofa-beds, a dining room table for breakfasts and some dinners, and a washing machine. And everybody can eat what they want to for breakfast and eat early enough so that your group or part of your group can get out early. A hotel or B&B breakfast, while seemingly convenient, will slow you up in the morning and you’ll only get out and about in late morning just when everybody else is pouring into the city — plus, it’s much more fun to go grocery shopping or to a street market or deli shop or fruit stand or coffee bar. We have never had any trouble renting our 40+ Airbnb apartments in Italy, partly because I am very careful to read reviews, examine photos, and whenever possible choose a Super-host. The trouble you will have, because I did, is finding apartments for 10 people. I found my family two apartments within a 5 or 10 minute walk from each other, in both Venice and Rome, and you might need three.
Here’s what our 6 and 9-year-old grandchildren loved in Venice:
I made them scratch-off “I Spy” treasure hunt booklets with photos of things in Venice to look for. I got the scratch-off stickers on Amazon, but you could just print up photos and the kids could cross a photo out when they see the thing.
They adored discovering pizza with french fries and hotdog slices on top (sounds awful, doesn’t it?)
Get a vaporetto pass and the Che Bateo app — we all loved sitting in the outdoor front or back parts of the boats, and the kids enjoyed “beeping” our passes.
Visit to Torcello in the early morning, especially climbing the belltower, but even the mosaics in the church. Nobody else was there.
We found almost every playground in Venice.
Glass blowing demo at the Glass Cathedral was cool and had no sales pitch or attached scary glassware shop — you can all see plenty of little fragile glass goodies by window shopping from the sidewalk.
iPads and Boogie Boards were a blessing many times
The Bovolo staircase was a hit with both kids
Burano was great
Rialto fish market
Surprisingly, eating long lunches in restaurants, even though the 6-year-old is quite picky. We took plenty for them to do, such as coloring pages to color, mazes, I Spy games, etc. all printed up from the internet. Plus colored pencils. The restaurant owners and servers were just wonderful with the kids and there were no problems ordering plain pasta with butter and cheese for the picky eater.
We knew better than to take these two to museums or art galleries but they did like the Leonardo d Vinci interactive museum and brief visits inside San Marco and other churches. It depends on what your kids are like — our own two sons when they were kids would have been fine in museums, etc. for maybe 45 minutes.