My two cents: Fly in and out of FCO and take the train to Venice right from the airport (you can do it without a transfer). It's a 4 hour train ride and you can all sit together comfortably. There are two trains a day directly from FCO to Venice (through Florence, Bologna and Padua) - 11:08am and 3:08pm, but you should confirm these for your dates instead of relying on me here. You'll want to time your TATL flights accordingly (see below).
If you all really want to fly together, you'll all need to get to a hub, and I'll let you google those flights yourself. As far as the flights into FCO, you probably want ~2 hrs before the train just to be safe. These get you to the early train:
https://www.google.com/flights/#sear...;ti=l0000-0908
These get you to the late train:
https://www.google.com/flights/#sear...;ti=l0909-1308
Then you can get your train tickets on the Trenitalia website.
As a practical matter, if you plan to sleep on the flight and can actually sleep on a plane, I'd suggest leaving from ORD for a slightly longer flight. If you just want it over with, the fastest is from BOS - but by the time meal service ends and they turn the lights down, you really only have ~5 hrs left before landing, as the total flight is just under 7 hours most days.
It sounds appealing to do an open jaw, but you can get to FCO from most places in the US, and the train station is connected to the terminal by skybridge. It's so easy. But if you do want to go to VCE, the few nonstops would be out of PHL, JFK and EWR. From VCE, you still have to take a boat or a water taxi to the island, whereas a train will take you right there.