I do not think there is a better youth program than RCL's for children (particularly the ages of your 2 children). The Voyager Class ships are also perfect for them - In addition to a stellar youth program AND facilities, the ship itself offers all sorts of other venues to keep the children (and the parents) happy.
As for the program: The 3-17 yr. olds are divided into 5 age groups. One of your children would be in the Voyagers (ages 9-11) and the other would be in Navigators (12-14). Each has a separate daily program and then there is always "The Living Room" and "Fuel" the 2 teen hang-outs.
I did a mega-review of the Vision of the Seas, Feb. 1, 2004 and included a daily program for a typical day at sea for each of the age groups in the Adventure Ocean Program. Although it is not a Voyager Class ship the acitivites and schedule are somewhat similar fleetwide - so that would at least give you a fair idea of what is in store for the children. Here is a link to the review (just scroll down to the pictures of Adventure Ocean and you will see the links to the individaul daily programs):
Also, if you check out Tom Ogg's mega-review of the Navigator of the Seas and Joanie Ogg's brand new review of the Explorer of the Seas on the CruiseReviews.com site you will see some great pictures of the Youth facilities onboard the Voyager CLass ships.
Hope that helps-