I vote 3 nights Venice (30, 31, Nov 1), might want to look into a "Venice in a day" small group tour, or at least the Doge's Palace and St Mark's or something like that.....that will hopefully be over in the afternoon, spend the evening wandering the pathways, let the kids help get you lost. Second day, a visit to an outlying island, then back to do a late afternoon/early evening Row Venice lesson (or their chichetti tour!). A food tour the first full day in the evening might be good (instead of the getting lost part) to help you feel like you're exploring, but with a purpose. Part of the fun of Venice is to enjoy the surroundings.
Then take a train to Rome, there you will have the nights of Nov 2, 3, 4, and 5. Gives you 3 full days. Arrival night you can take a food tour (Eating Europe was just recommended to me by some friends and I have seen people on here suggest as well), then day 1 Ancient Rome (Coliseum, Forum, Palatine Hill), day 2 Vatican, and then day 3 maybe some outdoor activity? Visit the Appian way? Catacombs? If you can't think of anything, then you could take a day trip to Florence or Ostia Antica.
I think having tour guides to make the history or art or whatever you are experiencing come alive for the kids might be a good idea and they will probably enjoy it more. The evenings will have plenty of time to wander and enjoy Rome.