P.,
I have a five year old son and in the last 12 months we have been on two cruises with him: Royal Carribean and Disney. You know Disney, so I won't comment on that, except to say that my very good friend just cruised with them (her 7th cruise with them, mind you) in August and really felt that they have diminished in terms of service. Check any boards with Disney to get plugged in to see if this may be a common perception among returning cruisers.
I cruised with Royal Carribean in March 2009 and I was very impressed. The food was terrific (service was spotty compared to Disney in Oct 2008) and the price was much cheaper. The most surprising part? The kids club was better on RCL than Disney. Unbelievable, I know, but it was my experience. I couldn't get my son out of the kids club. The kids club facilities weren't as nice as Disney, but the staff and activities were incredible. He got a lot of attention and brought many interesting projects back to the stateroom.
Carnival? I would go with your instinct. The agent is crazy if she thinks it's not the party scene. It is. They haven't changed. I wouldn't go on that line even if it were without my little guy. I would ask another agent in your area.
Good luck, and enjoy. We're planning another RCL cruise for Jan or Feb. to let you know how much we enjoyed it. Helps that it's a lot less than Disney-too. Then again, how can you beat Castaway Cay??