They will have a great time on the Grandeur of the Seas. Royal Caribbean has something for all age groups going on every day. If you visit:
http://www.cruisereviews.com/RoyalCaribbean/
and scroll all the way down to the bottom, you will find sample kid's programs that you can print out (you will need acrobat reader)
The drinking age on the ship is 18 for wine and beer and 21 for other drinks. In the ports it is much more liberal.
Tom