Family trip to Italy in December

You can eek a bit more time out of the trip by either flying open-jaw, OR, on arrival morning, hop straight on a train to Florence, putting all of your Rome time together at the end. Don't underestimate the time-saving value of reducing a hotel change! You usually can't check in early anyway, so a nap on a train, dropping bags, lunch, and a lot of walking outdoors in sunlight is the key to day 1.
There is no amount of days that is ever enough, just an overall balance to strive for--play it by ear on the day trips. Otherwise, looks lovely!

You could take a train immediately to Florence upon your arrival to Italy. Then do your entire Rome stay at the end of your trip.

Go immediately to Bologna by train on arrival. Three nights. Take the Italian Days tour. Thank me later.
Train to Florence. Four nights. There is more to see and do in Florence than you can do in a month, so keep your day trip options flexible.
Spend the rest of your nights in Rome. Taxi to airport the morning you fly home.

Edited to change Bologna to three nights to allow you a day for jet lag. Think of your time in nights, not days.

Don't get your hopes up too much regarding Christmas markets. They aren't so much a thing in Italy as in Germany or even France.

I agree with the others! Put all your days for your departure city (Rome) at the end! I always love taking advantage of continuing to travel when I arrive in country (and later check in time) to make one less home base change

I've seen some recent remarks about being disappointed by Xmas markets in Florence, but there are more than one so look around. Siena has an Xmas market but they tend to center on craft goods and run just on certain days: https://www.visitsiena.it/en/christmas-shopping/
But Arezzo goes all out: https://www.italia.it/en/tuscany/arezzo/things-to-do/event-arezzo-city-of-christmas
Lucca: https://www.madeinlucca.it/en/flowers-craftsmanship/christmas-markets-town

Having stayed in all three towns Siena is the best day trip town to visit unless the Xmas experience is your top priority then I would recommend Arezzo. Lucca is the kind of middle crowd - a better day trip than Arezzo but Xmas markets will probably be tasteful and subdued.

Have a great trip,
=Tod

Ahh you are all wonderful, thank you! Not sure why I didn't think of going straight to Florence or Bologna right away!! And I had hoped for an open-jaw trip, but alas to make the most of our points had to do Rome round-trip.

Does anyone have thoughts on being in Bologna during the week vs the weekend? (I read a post somewhere that weekends were better, but not sure why)

Speculation: In the past Bologna was probably more a business destination than a tourist destination, which might have meant lower hotel rates on weekends. I believe there's more tourist traffic there now, so it may not make much difference. And if your trip is late in the month, there may not be much business travel going on, anyway.