We will be traveling in March to Venice, Florence and Rome. I would love recommendations on where to stay if you have any favorite places. We will be two adults and two kids (11 yo and 9 yo). Thank you!

What is your budget?

Since you presumably want a quad room in each destination, I'd start by searching booking.com. You can search by date, budget range, and number of guests per room. This will at least give you short(er) lists of those hotels that have quad rooms (because not hotels do). Once you have narrow the options, come back for comments on the hotels you've identified.

We’re hoping for a mid range hotel. We’re looking at these hotels so far. Anyone have experience with any of them? Also, is a room with a view of the canal worth the splurge? Is the noise level in the canal very disruptive?
Thank you!

Florence
1. Palazzo Guadagni
2. Firenze Number Nine
3. Garibaldi Blu
4. Il Guelfo Bianco
5. Palazzo Galetti

Venice
1. Hotel Antico Doge
2. Hotel Sant’Antonin
3. Ca’ Pisani Hotel
4. Hotel Ai Reali
5. Hotel dell’Opera

This is where I'd stay if I went back to Venice. Nice, quiet area away from the crowds.

https://www.dahotels.com/grand-hotel-dei-dogi-venezia/

We visited those same cities when our kids were 8 and 10, so basically the same ages. The hotels that we stayed at that had quad rooms were Hotel Miami in Rome, Casci Hotel in Florence, and Hotel Canada in Venice. The first two were perfect for us and I would absolutely stay at either one again. The one in Venice was doable, but not our favorite, but I would be willing to give it another chance depending on current reviews. We had a bathroom across the hall, which is not a big deal, but the drain was mostly clogged and it was one that the toilet/shower/sink did not have any separation so with the very slow drain the rest of it became an issue. That one did not have an elevator, in case that matters to you. Location was great though!