For Venice, consider good all terrain stroller or a lead for 18 month old. All those cute little bridges you see,, look like ramps, but are often steps, so you will be carrying stroller up and down, and canals are not in any way safety fenced.
Do not skimp on getting a/c .. Italy is so hot in summer, honestly, cranky and hot kids equal not fun for anyone.
Honestly, I have no idea where to go with such young kids, I mean, you could go anywhere, since all kids care about is parks, gelato, and being with you,, so pick what you like. Plan short visits to sites like Museums and maybe focus on gardens and parks.
I would be tempted to take a easy vacation to somewhere on the water for a beach holiday,, save Cathderals and Museums for a few years. Maybe Cinque Terra, maybe not,, hopefully someone else has some nice suggestions for beachy spots.