A few random thoughts:
* - Wow, that's a very ambitious itinerary with a 3-year-old. In your shoes, I would probably only do two or three cities at most.
* - Booking C&P, where available, will save the most points. But do so quickly, as category changes are coming as early as Feb. If you already hold a reservation, you pay the old rate. If a property's category falls, you can rebook at the lower rate.
* - Check with the hotels when booking to make sure you won't be charged extra for a third person in the room. I don't have a lot of experience with Italian Starwood properties, but I didn't have an issue with myself, wife and then nine-year-old at the Danieli and Westin Excelsior Florence. But that was way back in 2004.
* - The Danieli is about as central in Venice as you can get. However, I would opt for the Westin E&R instead since it is fewer points. Both hotels are situated very well.
* - The Westin Excelsior in Florence is my favorite European Starwood. Not quite as close to everything as the two Venetian SPG properties list above, it is still right on the Arno and a 5-8 minute walk to the Ponte Vecchio. If you have your heart set on visiting Pisa, I would probably also consider that a day trip from Florence and stay here two nights.
* - I would skip Naples. If you want to stay in the area, you're better off staying at a smaller non-SPG property in Sorrento or even in one of the Amalfi Coast towns.
* - I'd probably skip Milan too with a child, but that's just me.