disneymomma1979, my 3 kids LOVED the kids club when they were under 12--they would stay for hours. They loved making simple crafts, playing simple games, etc. It is very structured. Yes, it is very secure and you can leave them when you are in port. There is a nominal fee if you are gone during lunch time. Plus, they have a great drop off center over on their private island. WE checked DD and DS (3-5 age range) in in the AM and at lunch they didn't want to leave--but we insisted to do family stuff. They were busy making sand tunnels will all sorts of funnels, wheels, etc. I don't know what they offer for 18 months--was nothing when we cruised--but I hear that has changed.
I am sorry to say that at 12+, they really didn't like the RCCL kids clubs as much. RCCL tends to be open in the pm, but the teen club was not open/attended during the day. Rather they have some active/sports stuff--dodge ball @ 2pm, basketball tourney, etc., which my shy kids did not want to do. We actually (partly) chose NCL in Europe last summer b/c the kids club went thru age 12--guess what, DD12 did not go the entire week! However, I got the schedule and they did have secure, structured activities up thru age 12. I think 10-12 was together.
It's weird, but my tweens/teens all preferred
DCL more when they got older--both for the Movie theater and the staffed clubs with activities. DD16 loved making the zombie movie at the Vibe.
Now, I will say DCL Edge was not perfect. DD12 went 2X to do a craft listed on the Navigator and they were doing something different in the Edge. But, they still like the "younger" things they do in the Edge--cupcakes, pizza-movie, making bracelets, etc.
If you have outgoing or sports oriented kids, then RCCL teen clubs will be a good fit. I just don't think they work for shy teens as much.
We find that for our shy teens, we need to be more involved and do more family things---putt-putt, a Movie (most ships show at least 1-2 Movies during the cruise--some have a small movie theater), shuffle board, play a board game together, etc.
Bottom line--under 12--I would cruise RCCL without a worry. At 12+, I would have to think about it and really look at what my kids would do on the ship. On DCL a movie theater really helps with the 12-15 age range, IMHO. We have another RCCL booked for when kids are 14-15, b/c of great ports and great price. I made sure there was a small movie theater on board (Radiance class supposedly has them).