If we go this year then it will be DW, me and our 2 year old daughter. It would probably be end of September or early October so it shouldn't be high season.
We haven't ruled out the Lakes but I wonder when is best to visit them?
We just returned from a couple of weeks in Italy and took our 2 year old with us. We went to Venice and Verona and some other nearby towns...and Milan. Our toddler LOVED Venice and Verona. September is a wonderful time for both. We were going to visit the lakes too but were told by many people living there in Italy that it's too iffy now weather-wise, and my guess is that it will be iffy in September and October as well. Just a guess.
Milan is not one of my favorite cities at all; we went there to visit family. It's kind of hectic. If you book it into your trip, I would guess a few whole days there is all you need. The Last Supper (book way ahead) is there and the Duomo...and La Scala. But otherwise, not much...unless you like to shop a lot. It's a busy place and we would have left it off if we could; we didn't go to the lakes around there because it just wasn't the right season for hanging around them.
Rome could be extremely interesting to even a young child; the architecture is fabulous and interesting. Florence is gorgeous and easy too.
But still, Venice is like no where else, and I think children love all of the boats and music around. If you do go, it would be wise to get a pass (one, two, three day -- maybe a weekly too, not sure) for the vaporettos --- the boats that take you from stop to stop (in lieu of a bus); that way you can bring a light pushchair with you. There are tons of bridges in Venice.
Make sure to have some fresh gelato.