My DD did her first one out of the GF at age 4. She is 11 now, so that would have been 7 years ago. She has never been bored on any of them, but like anything, experiences can vary and much of it is probably based on the CMs you get. Your kids might have gotten either inexperienced CMs, or ones that were having a bad day.
The last cruise DD did was at age 10 and while she still had fun, she felt she was getting too old for them and hasn't asked to do another.
Thanks for the age info, I think this will describe DD perfectly! She is 9, but loves being the "big kid" so I know that is not a problem. She also loves trying new things, so I can totally see her going, having a blast, but not having to do it again. I appreciate that feedback!