Actually in Costa Maya there is a large swimming pool in the middle of the shopping area. You can sit at a table under an umbrella and let your kids swim in the pool. There is a beach there also but we never left the shopping area. The beaches along that coast are amazing.. it would be worth doing the beach day thing. Look into booking something on your own, they can be had for a lot less then booking with the ship. You might even just be able to take a taxi to a beach.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007