Lodging near Milan Centrale station...

2 nice hotels just 2 or 3 blocks from the main train station.
I've stayed in both and they are great.
Hotel Berna (see their site)
or
Hotel Garda - just across the street from Berna.
I found the Garda to be a bit less expensive, but both are perfect for connections
to the trains, and the metro station is on the corner.
Lots of places to eat around this area too.

(I believe both hotels are listed in RS Italy Book too.)

TS, one alternative you might consider is staying the night in a hotel right at or very near Malpensa Airport, and then getting a morning train 'from Malpensa to Florence. That would save you a lot of time and hassle with your luggage. There is a Sheraton right at the airport terminal, although it is a little pricey. I stayed a night at the First Hotel Malpensa about a month ago, and it was really great and very reasonably priced. It is a 10 minute or less ride from the airport terminal in their free shuttle. There are many trains from Malpensa to Florence every day, hourly or more, although they involve a connection at Milano Centrale. Don't know if there are any that go straight from Malpensa to Florence without any connection.

In my research for a trip I found that the hotels near the airport, even the big name ones, were less expensive than the hotels in Milan. They all had free shuttles to and from the airport.

I like Hotel Garda and I've heard the Berna is good, too. Since you need to take the train, I would suggest spending the night closer to the station than to the airport. Both hotels are a 5-minute walk from the station entrance, very reasonably priced.

I stayed at Hotel Garda last November, and paid 65 euro for one night for a double. The hotel was fine, other than a minor annoyance of having to pay for WiFi. It was clean and a close walk to the central station.

Hotel Florida. 2 blocks from the station.

As you are arriving late and will be traveling for the majority of the day, be easy on yourself and consider staying at a hotel near Malpensa (I recently stayed at the Holiday Inn, found it very modern and comfortable, free scheduled shuttle to/from the airport) then catch the train from Malpensa to Centrale the next morning. Otherwise you will be getting in late, having to catch that same train from Malpensa to Centrale, finding your hotel, trying to check in, getting settled, etc.

Here's another vote for Hotel Florida. We stayed there in September and especially enjoyed their breakfasts. It's in the RS Italy guidebook.

Suggestion: stay on at an airport hotel, and take a train to Milano Cle. (or Milano Pta. Garibaldi if you are using Italo trains) next morning. Much easier with kids.

Quad rooms are uncommon in Italy, you'll probably have to pick two adjoining rooms that can be linked by opening an intermediate internal door.

Enjoyed the Berna, lovely staff and breakfast, but really not sure why you would come into Milan if you didn't have to. Hotel rates in Milan vary depending whether conventions are being held, so start checking rates now and book as you can.

We stayed one night at Hotel Star. Rooms chock full of cheesy african animal murals but overall not a bad place to stay for one night. From what I remember, it wasn't too far from train station; we also left the next day for Florence. I had a hard time finding hotels that would only let you stay one night; hotel star was one that would that was reasonable in price; I think under a 100 euros.

we use our frequent flier miles on American Airlines and usually fly out of Milan. We've stayed at Hotel Berna twice and really like it and its affordable (flight attendants stay there too- we chat with them in the hotel bar area). Its a quick 5 minute walk from the train station- and flat- which is nice when you have luggage to haul. 3 single girls and we all felt very safe walking around the area. You can also hop on the subway to the Duomo very easily.