We sailed in Feb. 2002 with our (at the time) 8 yr. old son. He went to Camp Carnival the first full day and stayed only about 45 mins. His problem with it was that he was grouped in with 6-8 yr. olds and felt that all the stuff was too "childish". There were far more 6 & 7 yr. olds. I asked the counselor if he could be bumped up to the 9-12 age category, but she said that was not allowed. Even without Camp Carnival, our son had the best time of his life. He went down the pool slide continuously, ate loads of pizza and soft serve ice cream, and enjoyed being treated special each night in the dining room. He made some friends on board all his age. None of them went to Camp Carnival. We are looking into cruising again later this year and possibly now that he is 9 and will be in the 9-12 age bracket he will think the activities are "cooler". Either way, he will once again have a blast I am sure. Your 3 yr. old will more than likely enjoy the camp and all the activities it provides for that age group.
Have a grand time on your cruise - you and your children will definetly enjoy it !