We just went on a cruise and left our three kids with my in-laws. The kids are 1, 3, and 6 with birthdays in July. We went in January, so all at half ages. The week we were gone was too long for my in-laws and too long for my kids. We had a great time, but we all agreed we couldn't do it again. Two weeks is much too long. The kids would just be adjusting to the time change by the time they got home, so chances are they'd be crabby if they went.
The tour guides cost tons of extra money, so I wouldn't feel obliged to go if you took the kids. Just get maps of the cities ahead of time from someplace like AAA that could give you advice, and have a fun relaxed time.
If it were me, I'd talk to my husband about how I'm feeling, and see if it would be acceptable to say a very kind and gracious thank you, but that you just don't feel this is a time in your life that you can do this.
If you do leave the kids, it's very expensive to call from the boat, but you might be able to call home from land a few times. I know of a few cheap ways to call in Italy, and I think they work all across Europe, if you want that information. Easiest is a prepaid phone card. You can purchase them really easily there.
I don't envy your position. Any decision you make is going to be hard. I hope you and your husband come up with a good solution that works well for you, and that the others involved can understand.