Tuscany base for 10 days with our 8 year old

Lucca and Pistoia are both good choices since the driving times to a variety of good things will be reduced, yet a train to Firenze would only be about an hour. Viareggio would be a fun trip from there as besides the beach and the 'boardwalk' there's a mini amusement park with bike rentals in the pines on the south side of town. Up the coast there's Lerici and Portovenere and Sarzana. Don't miss Collodi so you can teach the kid about bad friends: http://www.vacanzeinversilia.com/eng/collodi.html

Siena, Florence or in between will be more central to the areas of Tuscany you are likely to want to visit. Don't underestimate small towns for your child. He might be more likely to make friends with locals of the same age in a small village than in a big city. If he hooks up with some kids his age in a small town, he might decide that he'd rather spend time with his new Italian friends than with his boring adult parents at a museum. I have a good childhood friend from a small town in Tuscany where my parents were from originally and where I used to spend summers. He got married to an American and lives in Minneapolis. Every summer, since his kids were very small, he takes them to his Tuscan home town where nearly nobody speaks English. They have a ball with their Italian friends and cousins, going fishing and exploring the forests and doing stuff with a level of freedom that they aren't allowed to have back in Minneapolis. Every year they look forward to spending those summers in those small villages with their Italian friends, in spite of the language barrier, and hate having to come back home in August.

Thanks Roberto. Any particular town between Florence and Sienna that you recommend?

Check out the villa Santi Terzi in San Gemini Umbria. Just Google it. It is right in town.