In Florence, Hotel Rivoli has a family room that can fit up to five.
https://www.hotelrivoli.it/en/room-a...d-friends-room
For Venice, we choose to get two rooms at Four Points Venice Mestre. It is not in Venice itself, but a short train ride away. We choose it because (1) it is way cheaper than anything in Venice proper, (2) it is a fairly new contemporary hotel with good air conditioning and clean (3) it is within 6 minutes walking distance to the Venizia Mestre Ospedale train station for the train ride to Venice. If you have MR points to burn, this is a Category 3.
https://www.marriott.com/hotels/trav...venice-mestre/