If you want to get rid of them (yeah, yeah, they’re great, you love them and enjoy spending time with them… but sometimes a break is much appreciated), telling them that you’re just going to rest in the cabin might do the trick. At least for the first day.
But if you’re really just curious if they’re going to like the kids’ clubs or not, then I’m afraid this is something you really aren’t going to be able to plan out in advance. It sounds like they will, based on the activities they like, but they might just enter at the wrong place at the wrong time.
I can tell you a story about yours truly on her first (and, to this day only) Disney Cruise -
My parents dropped me off at the kids’ club on the second day. I had toured the kids’ club when we were in port and saw the pirate slide in the younger kids’ club (which my sister and I were in), and I was excited. Well, my parents dropped us off and they immediately took us from the little kids’ club to the big kids’ club (which wasn’t as exciting to a 7-year-old) to do an arts and crafts project. You’d think I’d love it because I really enjoyed arts and crafts - we made glitter frames, and Mickey Mouse came to visit us! But no, I was crying because I was a weird little kid and worried/disappointed that we were in the wrong place. My 7-year-old self couldn’t just “go with the flow,” so they beeped my parents (a great system they have with Disney - you will be immediately notified if your kids need anything) who had to come pick us up… apparently it had only been half an hour or so. Two days later, our parents encouraged us to try out the kids club again (there may have been some bribery involved), and we absolutely loved it. We stayed in the little kids’ club and learned dance moves for a farewell show, did the farewell show (and got free t-shirts… always loved free things, and I actually still have mine… I believe they stopped doing that, though), and then went back for free play (which is what I was wanting/expecting the entire time).
I think the kids’ club is structured now so that it is majority free play with optional activities. There’s also a new set-up so that the kids of different age groups intermingle with one another, so you don’t have to worry about siblings being split up.
I’d recommend that you have them try it out early. Not so much for the little kids, but for the 12-year-old, these kids form cliques pretty quickly, so he needs to get in there on the first/second day. And check the activities schedule to see what they’re doing at that time and try to set them up with something they’ll enjoy. And if they’re control freaks like me, make sure they know exactly what to expect as to where it is and how long it will last.