They are pretty strict, and the child will stay in their age group even if they have a birthday and age into a new group during the sailing. Also keep in mind the kids clubs are only open for three hours at a time with usually 2 hr breaks (I'm speaking mainly on sea days, port days have different hours). They won't be apart all day, and depending on your schedules, most likely will only go to kids club once per day; there's three sessions per day.
We just came back from Discovery. My child and his cousin were in the treehouse (age 3-8) and the other cousin was in the next age group up. We all thought it was good they were able to spend some time apart for a bit and tell each other about what they did in their club that day.
Btw princess does a great job with the kids club. The crafts and toys they give out are really nice and good quality.