I agree with all re xeroxing key documents. You should consider extra copies of your travelers checks receipts as well. My stepdaughter had her purse stolen last week in Amsterdam. We had insisted she take copies of everything - she had all new documents and replacement trav checks within 24 hours and the loss is now just another travel story. You might want to bring copies of birth certificates for the kids to prove they are actually yours. There have been some reports of "officials" questioning parents about the legality of them traveling abroad if it's just with a single parent. (The whole divorce/custody thing)

In Venice a few things; the Peggy Guggenheim Museum - for both a great museum and one of the only truly airconditioned places in the city. Drinks on the deck of the Gritti Palace hotel - at about 5PM - Expensive but cheaper than a meal, the view is great and the people and "boat" watching is better. Even though the price is outrageous you should not leave without taking a gondola ride, and if you have some $$ to spend, the glass factories at Murano will usually pay for a water taxi to take you over for a tour (and of course to buy). You're not obligated to buy and the glass demonstrations would be good for the kids, and the water taxi ride itself is very nice. Your hotel can probably set that up.

Have a nice trip.