Mikewdw we just returned from our fourth Disney cruise. We've had kids of all ages with us on them.
For me the biggest issue was waiting until the kids were 3 and potty trained. If they are under three or not potty trained, they can't go in the kid's clubs, only Flounders nursery where you are charged. You seem to have cleared that hurdle.
Even if you don't use the kid's clubs all the time, it's nice to have them as an option when you want to go to an adult dinner, an adult show, etc.
Having said that, there were two 18 month olds on our last cruise with us and they really had a great time. Their parents had to sit out some of the activities while the kids napped, or if it was after bedtime, but overall their parents will have some wonderful memories of the cruise. The kids won't remember it, but the parents will.
Phonedave is right, Disney does an incredible job of entertaining the kids. We really don't see them much onboard. There were times we made them come out of the clubs to do something with us.:lol:
Every night you'll get a list of the next days activities. My dd11's favorite thing was to sit down with a highlighter before bed and plan her next day.
I have a trip report going right now in the trip report section if you're interested in some more details or pictures.
Just ask if you have any more questions!