I wouldn't take a 3 and 4 year old to Venice or Florence - firstly they'll be heaving at Easter and secondly there is nothing there to interest them, so they'll be bored and whiney. If you want to go to Italy I would go to Rome, which my kids enjoyed. The Colosseum was surprisingly dull, but they loved scrambling around in the Forum, the ice cream was always amazing (and a good bribe, when needed), there are lovely fountains to search for and dabble in, a nice zoo, the food is child-friendly and Italian waiters are fab with DC - they always bring colouring pencils and paper and joke around. Plus, there are parks and a few playgrounds if you search around.
My top choice, however, would be Paris, which is great for kids of all ages - really good playgrounds, boat trips, natural history museum and lovely parks for strolling. We've taken our DC several times, the first when DS2 was only 18 months.
The other place I'd recommend is Bilbao, which we all loved. Lots of lovely outdoor space along the river, museums that are quite fun for DC in small doses, a funicular railway, good playgrounds and lovely pintxos (Basque tapas).
We're going to Valencia in Feb, which looks good. Lovely parks and the City of Arts and Sciences, which looks like it has plenty for young ones to look at and potter around.
If you go to Rome or Paris get the DK Rome or Paris with Kids guides. They are really good and help you to locate the nearest playground or ice cream stop wherever you are and detail what is in the various museums and things and what age groups they suit.