Personally, I would cross Venice off of your list. It is a magnificent place, beautiful and unique. But I can't imagine two children there for 10-days. I spent two days there and I could have seen spending two more, but it can get disney-esque in its intense touristy-ness. It can start to grate on you.
Also, the best thing about Venice in terms of food that I experienced were early-evening snacks (
cicchetti) eaten on bars and washed down with little glasses of wine or beer. It was immensely enjoyable to move from bar to bar and eat cured fish, fried things, little sandwiches, olives, etc. I did not see to many children, if any at all in these crowded bars. Beyond that, the best of Venice seemed to be in expensive restaurants.
If Italy and food is interesting to you, I'd suggest camping out in Bologna and making smaller side trips to places like Siena. The medieval vibe that your son would love is overflowing in those places and I think your daughter would like it too. The people are wonderful and the cities (especially Bologna) is vibrant and varied. But, most importantly, the food is impressive from every angle. Street pizza, delis, bakeries, street markets, mom & pop trattorias, etc. there's so much to eat and cook that you'll never eat the same meal twice and you'll love every one.
When we left Bologna, I said to my wife, "When we have children that are old enough to come to Europe, we're renting an apartment here with a really nice kitchen for a couple weeks."
Best,
Michael