Hi Melinda!
Awesome 5th birthday celebration!!!! The Disney Fantasy is stunning, and you're going to marvel at everything..the decor, the luxury, the service...I mean EVERYTHING!
I'll be honest, this is a tough one to answer, as both are wonderful. I'd have to say I'd go for the Dolphin Encounter in Costa Maya.
Dolphin Discovery Cozumel (from Disney Cruise Line's website):
The dolphin encounter takes place in an enclosed open sea area where Guests stand on a three-foot deep, metal grate platform (varies with tide). Guests are encouraged to wear water shoes.
Guests must be able to descend 12 steps into the water and be able to stand on the platform throughout the dolphin encounter.
I'm not very keen on the idea of having my 5 year old standing on a 3 ft deep platform in the ocean. Remember, if the tide is rough, so will standing on that grate. Please also consider your son's swimming skills. Remember, the tide can be strong.
I hope this helps you decide Melinda. Please let me know if I can help you with anything else. Thanks for visiting the Moms Panel. Bon voyage!