I plan to have my twin DDs stay with the 8-9 year olds in September, even though they will turn 10 on the second day of the cruise.
We were on the 7 day cruise last year and some of the kids in the 10-12 year old group were a bit too mature for my girls and I would prefer for them to stay with the younger group. Next year, they can go to the older group, but I don't want them to be the youngest in that group.
I think you have to consider your children and where they would best fit in. Frankly, many of the 8-9 year old activities are with the 10-12 group, but they do do some different things. I checked the navigators on Barb's website and I know my daughters would enjoy the activities for the 8-9 year old better (hanging in Common Grounds with the 10-12 would be boring for them this year, but maybe in another year they would find it interesting).
If you are on a 3 day cruise, you might find that they spend very little time at the club. There is SO much to do and so little time to do it! But on the 7 day cruise, we did use the club quite a bit.
I agree with the suggestion of looking at the navigators on Barb and Tony's website and deciding from there. That is how I knew my DDs would prefer the 8-9 year old group this year.
DJ