Or first cruise was when my DS was seven years old (almost 9 years ago; where does time go?). This was our first big trip with him and we thought he would enjoy the parks more so we went with the 4 nt. land/3 nt. cruise.
We could not get him out of the club he was having such a blast. When we were at the airport to go home we asked him what his favorite part of the vacation was and it was the kids club on the Wonder. I can't say that every child will be this way. I know a family that could not keep there children in the club they only wanted to be with mom and dad. My theory is our DS had been in day care and their children always had one of them around. The husband worked during the day and when he was home she went to her job in the evening cutting hair.
I would go with the four night at least, even this will probably feel too short. And give some thought of doing a split between the parks and a cruise. Land first followed by relaxing on the Wonder. I'm sure you'll have a blast whatever you choose.