the cruise lines are pretty strict about the age groups. If she isn't in his age group for Camp Carnival chances are pretty slim that they will bump her up into his group. The activites are geared for the specific age group. You might have a chance if only a few kids enroll in her age group, and they might combine groups. It can't hurt to ask when you go to enroll her in camp.
my 14yo son was in camp. Kids this age can sign themselves in and out of camp on there own. We found that once all the kids hooked up they kinda did their own thing on the ship and didn't spend much time in the camp itself. I don't know about specific camp activities regarding them being together but they did allow our 5 yo to go into the 6-8yo camp cause his brother was in it. It was a tough sell but they did it. i think a 13yo in a 14-16 camp could be an easier thing. Regardless you kids will have a great time.
RCI Majesty of the Seas Aug 2007
Breckenridge ski trip Apr 2007
Carnival Fascination july 2006