our daughter has cruised at 8 weeks, 2 years and 4 years; and had long plane rides every year on other non-cruise vacations. (and I'd note that the only time in her life we went on a weekend trip without - that the first week we were back - was the only time she'd ever had a problem sleeping - none while we were gone and she stayed with her cousins - only when we came home - I think she waned us to know she was mad we didn't take her with us! but I digress.
We have been fortunate all the cruises have been smooth. This summer she loved the kids club, but she's 4now.
But I think travelling with small kids on a cruise is easier than other sorts of travelling because you unpack once and you're there. Esp. on Disney the littlest ones can use FLounders (not always available on other ships). On RCL when our daughter was 2 we hired a babysitter (who was a crew member - we hired her through the purser's desk) - she came every night for 2-3 hours - our daughter came down for dinner - ate and we took her back to the cabin and we enjoyed the remainder of our dinner and then went for an after dinner drink or lost some money in the casino. I know many people would not hire a total stranger as it were - and although the cruise line would deny liability - they certainly are going to check out who they let "hire" out to guests (and had we been uncomfortable with her with didn't have to keep her - and she told us that if she hadn't liked us she would not have come back! it was extra money for her but if the kids or family were unpleasant then she wasn't going to put herself through the trouble) (My niece for example would never have hired someone from the crew as she wouldn't have "known" them - well, we got to know our babysitter quite fast - you can learn alot, I think, in a 15 minute conversation - if you go on the basis the person isn't going to be a sociopathic liar!)).
Anyway, we would plan what we wanted to do....but know in the back of our heads that our daughter's temperment or hunger pangs or need for a nap might alter what we wanted to do. We've always felt her schedule should fit into ours as much as is possible (and obviously understanding babies need naps, extra feedings, etc.) so I think parents have to look to their own personalities first to determine if cruising with a toddler will be enjoyable. (We have friends who claim it takes them and their toddler son at least a week to recover from the time difference following a flight from the west coast to the mid-west - 2 hour difference - maybe it is true - but I just can't imagine it to be - needless to say - that trio would not do well on a cruise with their son as a toddler - it might involve time changes, meal times at different times, - they are on a very fixed schedule and cruises and vacations in general would be frustrating to them unless by some chance their schedule fit the cruise schedule.