What's been stressed to me is to get your kids (of any age) on embarkation day...especially the older ones, make sure they go to the late evening mixer. (And they have to stay there for it. No "two minutes and done".) Fought this battle with my daughter last month. The open house at Vibe was a turnoff because at that hour, it was just kids who didn't want to be on a cruise. Took quite a bit of work to get her back in for the mixer, but eventually she relented and went in. We waited a few minutes to make sure she stayed, and while we waited, we watched another family take their daughter in, but let her come out after two minutes. My daughter not only stayed to the end, stayed out well after midnight every night and still gets text messages from the group from Vibe.
If the kids think this is part of the cruise experience right from the start, it'll likely go better. And if you have plans depending on them staying at the kids club, plan it for later in the cruise if possible so you can cancel if it's not working out. Let them know that you are still going to have family time, but there's also other opportunities to have fun that are not dependent on mom and dad watching them 24/7. And those are the best cruises because it becomes a true fun vacation, because parents deserve, if not need, a break and have some individual or couple time along side family time. There aren't a lot of vacations where you can do that, but that's part of what makes cruises so desirable.