+1 for Stresa
And bring some good insect repellent...
Consider staying in a 3-star hotel and paying a little extra for a room on a higher floor with a view of the water, rather than a 4-star if you're going to be staring at trees (which are full of flies).
Venice and Florence have tourist trap areas. If you eat on the main squares you will end up paying for the privilege, plus watch your change carefully! You might be expecting change and they might be thinking you are American and super generous\clueless with tipping.
The food will likely be cheaper and better down the side streets - and more authentic. It's worth checking lonely planet \ rough guide for their restaurant recommendations. You can sometimes pickup a copy in your local library, or in local charity shop.
Mid-July will be peak tourist season so the museums of Florence & Venice may get very busy - and probably not ideal places for a 5yo at best of times.
Try to hit them early or late in the day to avoid the crowds.
In Florence, visiting the Boboli Gardens is a nice day out.
But check the opening hours online before visiting anywhere... for example, the Boboli Gardens are closed on the 1st and the last Monday of each month. (Plus if Fiorentina lost Saturday's match by an uneven number of goals, or somesuch)
It's really worth trying to stay in Venice itself overnight so you can enjoy it in the dark, everything looks different illuminated.
Taking one of the water buses out to Murano is a nice trip on the water, and not expensive.
Again, you might have to go for very basic accomodation.
I've heard Verona is lovely but haven't been personally, think you could fit that in en route between Milan and Venice.
Finally, to get yourself in the mood, set your UPC\Skybox to record Italy Unpacked on BBC2, think it's being repeated in the week after Easter.