Boredom would be my concern as well. Spoiling them with luxury is not an issue I think, since a 5 year old is not going to see this as luxury, but tedium--they would rather have a hotdog than caviar. Waiting a couple of years would help, I think 5 is awfully young, 7 or 8 would be better, and perhaps a more mainstream cruiseline, with more organized children's activities would help as well.

I only experienced a problem once, on the PG. We had booked a deep-discount post-9/11 special, and had inadvertently booked on Easter weekend (would have anyways.) It was really only a minor hassle, the kids completely took over the tiny swimming pool, so that despite the very hot weather in April, it was basically unavailable to adults. At least it kept them occupied they were otherwise nicely behaved.