Don't drag them around the galleries or they (like the adults) will get culture overload. Try looking online to find a particular picture in a particular gallery; research its history and take them to see that, and walk past all the others. Better to look at one in depth than do the yawn-worthy trek that will bore them to tears. Preferably find a pic with animals in it - or lots to try and spot in the background. When I went, my DD had just completed a term at Venice university, studying History of Art, and she did just that - she marched us past everything in one particular gallery and said: "This is what you need to see!" She knew everything about it and it was fascinating.
St Marks Basilica is of course awe-inspiring, and will be more so for little ones because of the size - but make the visit brief.... although you might find yourself queuing to get in this time of year. Let them climb to the top; and also to the top of The Campanile di San Marco - they will love that, especially if someone can take a pic of them waving from below.
My most exciting moment was coming out of St Marks walking on tables as the Aqua Alta had descended and the square had flooded while we were inside. Not good news of course, but very interesting.
That's quite enough culture ........
Can you afford a gondola ride? - or if not a boat ride to Murano to see the glass-making?
Cafe to watch pizzas being made?
They might just enjoy running round the back streets and crossing the bridges, as this is a novelty. If you head for the back streets and the the Jewish Ghetto there are cafes that do not charge as much.