No matter what you choose, you'll obviously be a tourist from somewhere. Based on what I've seen in airports, and on what our son does with his family, it would probably be easier for the 10-year-old to keep up with a backpack. The rest of you could do either.
The most important thing is for everyone to pack light and to remember that your group will travel at the pace of the slowest one of you. The less encumbered you are with luggage of any kind, the easier it will be for all of you to stay together, especially when getting on and off trains.
I don't understand the cobblestone comment. Whether you have a 2-wheeled bag or a spinner, cobblestones aren't that big of a deal. With the latter, you just tilt it and roll it on 2 wheels when needed. If it's been a long time since you've been in Europe, it may surprise you to see that most adult Europeans use rolling bags.
For extensive discussions on backpacks vs. spinners vs. 2-wheeled bags, go to the Rick Steves Packing Forum: https://community.ricksteves.com/travel-forum/packing
Whatever you decide, you're going to have a great time. Our son took his 2 older daughters to Paris when they were 10 and 12. The younger one actually enjoyed the trip planning and the trip the most.
Edited to add: There are no issues with visitors having wheeled bags in Venice other than the fact that the bridges have steps, so you have to maneuver over them. You can Google about the topic, but here's a little exchange from the Italy Forum about it: https://community.ricksteves.com/travel-forum/italy/no-roller-bags-in-venice