Are you doing this through Carnival or an outside operator?
This is not a "here's your tank, jump in" kind of thing. even the beginners scuba is about 30 minutes or so of classroom (or sitting on the boat listening) before you even put the gear on. No offense, but a dive boat or shore dive is no place for a child. Who is going to watch them when you do your diving?
You will most likely go to 20 to 40 feet down. If this is done from a boat, that is pretty deep for even an adult to snorkel in. No, it would be unfair to the other people and to the operators for you to bring a child along. Not to mention dangerous.