Thanks for the information. As a matter of fact, Costa replied to an email I sent asking the same question and here is their response:
Thank you for your recent email.
Yes, we do have some promotions where kids sail "Free", actually they would pay just the nominal government fees. You can get more details by calling our Reservation Dept. at -800-247-7320.
Thanks for your interest in Costa Cruises.
I would have to say that this is encouraging, thank you for your interest, Heinbloed.