Here is our 2 cents about our cruise on August 25th 2007.
I am a 30 something housewife sailing with my 30 something hubby and our 4 year old daughter going from Venice to Barcelona.
We booked our own air flying from SFO to London and stayed there a few days and then caught a cheap British Airways flight to Venice. Upon arrival, we took the bus to Venice and then bought a vaparetto pass for 72 hours and went to our hotel. We stayed at Locanda Art Deco. It was a great location for a decent price. Remember that in Europe most places charge you for the 3rd person even if they are a child, no be prepared. Some American chain don't, but then their prices tend to be higher... We spent 3 days in Venice and were busy every moment soaking up the sights and indulging in gelato. I thought 4 days to see Venice would have been perfect, there are too many churches, museums and general ambiance for just a day trip or 2, arrive early and enjoy! It's gorgeous without cars to worry about, your kids will love traveling by boat everywhere.