We went on 3
DCL cruises and then tried the Oasis last April. Our kids were 9 and 11 at the time of our Oasis cruise.
Here's what I can tell you:
1. My 11 year old son enjoyed the Oasis kids club more than the Disney kids club - it had more interactive activities and the counselors were much more engaged. We went on the Fantasy when he was 10, and he was getting bored (gasp!) with the kids club. But if you think about it, the kids club goes as young as 3, and the kids are pretty much running around, wandering from room to room doing whatever they want - if you have an older guy, he may feel totally out of place. So this is a drawback on the wide age range. Sure, we could've had him go to the tween club but at the time we didn't want him to have roam of the ship. So, if you have a "borderline" older kid, Royal may have a better kids club that is more fun.
2. My 9 year old daughter, by contrast, hated the Royal kids club. She felt it to be stuffy and small. The kids were engaged with the games that the counselors wanted them to play, and that was it. If the child didn't want to play the game - well, lets just say they could either play the game or watch others play the game. That was it - my daughter is more of an independent creative type, and she didn't always feel like playing a game.
So - here's this - we dropped her off the first or second night of a 7 day trip and she started getting very seasick. Well guess what? THEY HAD NO WAY TO REACH US! We weren't given pagers, there were no wave phones - so they left a message on our cabin phone. Are you kidding me? We had no clue our daughter was up in the kids club turning green. So - just be aware... I think they do have pagers, and we didn't know to ask for one. So if you do use the kids clubs - be sure to ask for a pager if you don't get one of the wave iPhones (they only have a limited amount and there is a fee for it.)
Okay - now, your question about being "divided in the kids club." Royal will gladly allow kids to move DOWN to an age group, but they are hesitant to move younger kids UP. And once they make a choice, they have to stick with it for the duration of the cruise. So, when we went on our cruise, my 9 year old daughter had her best friend who was 8 and her best friend's daughter was 5. So, the 5 year old was able to move up to the 6 - 8 year old group. My daughter moved down, but she totally hated it - so she moved back up, and evidently it was a problem when she wanted to go back down again. So, the moral of the story is - make a choice and stick with it. But realize that the counselors are VERY involved in leading the kids in whatever activity is happening - so if the child moves down, they may very well feel out of place and bored.
Also, one other thing - keep in mind that Royal starts charging $ to watch your kids after 10 p.m. We found on the Oasis that many of the adult parties STARTED at 10 p.m. It was definitely a late night ship.
In short (not that you asked) we would go on the Oasis as a couple, and we would go with older kids - but I don't think we'll take our kids until they are both fully teens.
I thought the entertainment was upper level as well - very sophisticated. Aside from the diving show and the ice skating show, it wasn't what I would call very kid friendly. (But it was still very good.)
HTH